PRACTICAL JULY 1990 £1.50 ELECTRONICS

PRACTICAL JULY 1990 £1.50

ELECTRONICSSCIENCE AND TECHNOLOGY

MORSEDECODER

DIY PCS

MOBILE RADIO

HOME BASE

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Z -MATCH - Takes the drudgery out of R.F. matchingproblems. Includes many more features than the standardSmith Chart.Provides solutions to problems such as TRANSMISSIONLINE MATCHING for AERIALS and RF AMPLIFIERS withTRANSMISSION LINE TRANSFORMER and STUBMATCHING methods using COAXIAL LINESMICROSTRIP, STRIPLINE and WAVEGUIDES. Theprogram takes account of TRANSMISSION LINE LOSS,DIELECTRIC CONSTANT VELOCITY FACTOR andFREQUENCY.Z -MATCH is supplied with a COMPREHENSIVE USERMANUAL which contains a range of WORKED EXAMPLES£195 for PC/XT/AT etc

Write or 'Phone for full details:-Number.One Systems Ltd. I The CAD SpecialistsREF: PE, HARDING WAY, SOMERSHAM ROAD, ST.IVES, HUNTINGDON, CAMBS, PE17 4WR, ENGLAND.Telephone: 0480 61778 (6 lines) International: +44 480 61778 ACCESS, VISA, AMEX Welcome.

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NEXT MONTH

We've an exciting new item of digitaltest gear that's an ideal add-on for youroscilloscope, increasing the number oftraces from a single beam to eight, andproviding 8 -bit sampling and 16K ofmemory storage for looped repetition.Audio analysis is the purpose of our AFOctave Measurer, designed to performrapid assessment and multiple displayof signal frequency content. And still onthe subject of testing, we've an easy -to -built analogue voltage probe for use withdc and low frequency circuits. All thisand more to show you how to enjoyelectronics to the full.

* BE EQUIPPED TO PASSTHE TEST

* DON'T MISS OURFEATURE FILLED

AUGUST 1990 ISSUE

* ON SALE FROM FRIDAYJULY 6TH

PRACTICAL ELECTRONICSVOL 26 NO 7 JULY 1990

CONTENTSCONSTRUCTIONAL PROJECTSMORSE DECODER by John BeckerFor amateur radio students and the generally inquisitive, our semiintelligent lcdmodule helps un-dash the Ts and un-dot-dot the Is.

REMOTE POWERED MICROPHONES by Vivian Capel ....27The popular Electret microphone gives years of excellent service, until the internalbattery gives -out, sometimes with disastrous results. Our phantom could help youbeat the jinx.

AC MOTOR CONTROL by Andrew Armstrong 43As an answer to an Ask PE query, we describe one way in which a variable speedcontrol unit can be used with ac series wound motors in the 1-2kW range.

12

SPECIAL FEATURESDIY PCS by Robert Penfold 20Authoritative advice on assembling your own IBM PC compatible computer from anexpert who has built his own, and written this feature using it.

HISTORY OF TECHNOLOGY - HERTZ by Ian Poole 31A foremost researcher of his time, Hertz proved the existence of electromagneticwaves, laying the firm foundations upon which radio technology was built.

HOME BASE by Ian Burley 32Philips launches CD -I, British Aerospace announces Microtel. BYPS rabbits -on aboutcordless phones, and will Tron provide the key to city intelligence?

MOBILE RADIO by Mike Sanders 35The popular concept of mobile radio is that it's a vehicle -mounted system with a fixedbase station, but hand-held and back -pack systems also qualify. We examine thehistory and the technology.

BASIC ELECTRONICS - PART SEVEN by Owen Bishop 49More on transistors, and a modular demonstration of how to score a good try on thetouch -line and never be off -side for frost or fire.

WINNING BONANZAS by the Editor and a few readers 52The winners of our robotic's books competition have been drawn. Some ought tohave been hung and quartered as well for their limerical worse -verse!

REGULAR FEATURESEDITORIAL by John Becker - Heritage 9

LEADING EDGE by Barry Fox - Insecure cells 8

SPACEWATCH by Dr Patrick Moore - Brown dwarfs 46

TRACK FEEDBACK - Readers' letters, and a few answers 51

INDUSTRY NOTEBOOK by Tom Ivan - The megacellphone 57

PRODUCT FEATURESNEWS AND MARKETPLACE - what's new in electronics 4

SPECIAL SUBS OFFER - saving pounds on PE 10

BOOKMARK - the Editor's browse through some new books 41

ARMCHAIR BOOKSHOP - haven for practical bookworms 58

PCB SERVICE - professional PCBs for PE Projects 60

HOW TO DO IT - the route to better board building 61

ADVERTISERS' INDEX - locating favourite stockists 62

PE TAKES TECHNOLOGY FURTHER - BE PART OF IT!

N EJW\SMAPLIN'SGLOBAL WARNING

SPACE AGE STEREOSToshiba are introducing a brandnew series of personal stereos

designed with an image for the 1990s.Due to be introduced in July ataround £49, the KT452 I featured inthe photograph will be one of the firstpersonal stereos on the market tooffer digital tuning at less than £50.

Toshiba's 1990 personal stereorange has been designed to build onthe solid foundations which theyconsider were laid by last year'swinning collection. They intend thatthe stereos will consolidate thecompany's ever strengtheningposition in this market sector.

All ten models are styled in a'new spage age' Titanium finish,perpetuating the 'family' themewhich significantly contributed to thesuccess of their 1989 range.

In the new range seven of the tenmodels feature Dyna-Bass earphoneswhich have been specially designedto enhance bass frequencies. Fivemodels feature autoreverse, five useDolby, five have a three -band graphicequaliser, and two have digital tuning.

The top model, costing around£69, is the KT4551. Its featuresinclude an am/fm 2 -band radio,digital tuner with ten -station randompresets, 3 -band graphic equaliser,Dyna-Bass, fast forward and rewindcontrols, metal tape, and a belt clip.

At the budget end of the range isthe KT4 III, costing arourid only £19.which includes Dolby noisereduction, metal tape, a belt clip, plusRR and REW controls.

Watch out for these new models atyour local Toshiba stockists.

The new World Calender ClockAlarm from Maplin makes it easy

to keep check on time and datesworldwide. This superb clock hasreally taken your Editor's eye ( hemight even get himself one for hisbirthday!), for it gives the correcttime at any one of 35 cities aroundthe world. Just pressing the touchsensor map will identify the time -zone band and reveal the correct time.

The smart desk -top 24 hour clockcalender shows the date, day of theweek, month and year from 1988 to2037 on an 85mm wide lcd display. Aworld map measuring 170 x 75mm ismarked with a total of 27 time zones,

and just by touching one of thesezones the relative time will bedisplayed. In addition, a special zoneallows for the British Summer timeone hour offset. Powered by just twoAA size batteries, the alarm can beoperated as a time of day reminder ornormal alarm clock.

The World Clock is available fromall of Maplin's nationwide shops, orfrom their mail order offices. The costis only £24.95, including vat, but notethat a small carriage charge will bemade for mail order.

For further details contactMaplin at PO Box 3, Rayleigh, Essex,SS6 8LR. Tel : 0702 554161.

CATALOGUE

DATABASEOur browse through recently

received literature

Infotech/TIS are mail order distributors and wholesalers of HeinemannProfessional books. They also have a wide selection of books, manuals anddata sheets relating to many aspects of technology. Their latest I2 -pageloose-leaf catalogue lists subject matter for tv repair and service, video andradio, audio and domestic, computers, electronics, satellites, and even vehicle

wiring. They also have a selection of books for such diverse subjects ashypnotism and betting. The company belongs to the Society of TV, Radio andElectrical/Electronic Engineers. You'll find their information highly useful.Infotech/TIS, 76 Church Street, Larkhall, Lanarks, ML9 IHE. Tel : 0698884585.

Mauritron have sent in their latest catalogue. The company specialise inthe supply of workshop service manuals for almost any item of equipment."No matter what it is or what its age", say they. (They don't seem to have dataon the Commodore Pet 3032's vdu driver circuit, though. Mine's died, and Iwas disappointed that Mauritron couldn't supply data on it.) However, theysay they have a vast library covering over 100,000 different makes andmodels of the earliest vintage valve wireless sets to the latest tvs and videos.But why not those for my machine? Mauritron also run a technical societywhich they describe as a loosely knit, worldwide association of people withsimilar interests, amateur and professional, centred around theelectrical/electronic industry. Mauritron Technical Services, 8 Cherry TreeRoad, Chinnor, Oxfordshire, OX9 4QY. Tel : 0844 51694.

Cricklewood Electronics have a good bumper catalogue. It's their 15th,and the first of 90's decade editions. It has a cover price of £1.50, includingpost, and you'll no doubt reap the benefits from this small investment.Cricklewood say that they have added around 2,400 items to the catalogue.which is partially due the company having bought out the entire stocks ofUSD Ltd. They have also branched out into hobby kits, books, microphones,speakers, headphones, video heads and aerials. In the new catalogue thereare over 160 pages filled and illustrated with information and prices of a verywide range of electronic products. In addition to the new ranges, their standarditems are there in good force, including transistors, ics, triacs, transformers.connectors, passives and tools. There is a very well itemised index at the endto help you find what you're looking for. Cricklewood Electronics Ltd, 40Cricklewood Broadway, London NW2 3ET. Tel : 071-452 0161.

4 PRACTICAL ELECTRONICS JULY 1990

N E -An/ -SMETERINGMAGNETICS

ou'll recall that in our May issue,our American correspondent

Wayne Green highlighted the possiblepotential hazards of low levelmagnetic fields. These hazards, hereported, could even be present in thefields associated with electricblankets. Following on from hisreport, Wayne has kindly arranged fordetails of an American magnetic fieldstrength meter to be sent to us.

The photo shows a metermanufactured by Integrity Electronicsand Research Inc, of Buffalo, USA.It's their 50Hz Magnetic Field Metertype IER-I 19 and is for monitoringpower line, instrument, wiring andappliance magnetic fields. It hasmultiple range settings from 2 to2000 mil ligauss (0.2 to 200microtesla) and a narrow bandwidthof 11 Hz. The hand held digital meterhas a very readable lcd display thatincludes a low battery indicator. Italso has a programmable alarm, bothaudible and visual, and it's easy touse.

The field meter features astandard pushbutton backlight and astrip chart recorder output with anoptional ac adaptor. Its maximumsensitivity is I microgauss (0.1nanotesla). Linearity throughout its

50 Hz Magnetic Dosimeter

50 Hz Magnetic Dosimeter

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integPity EiccertnitiEN Its

range creates a 0.1% precision andrepeatability. Possessing anelectrically shielded case, theprecision instrument also comes witha shielded and grounded 8 inch longcoiled probe attached by coax cable.

The European distributors forthe IER-119 are conveniently basedin London. They are PerspectiveScientific, 100 Baker Street, London,W1M ILA. Tel: 071-486 6837, andask for Nick Clough.

BOB'S SAFER HOUSEWe're sure many of you watchedthe BBC TV program Q.E.D.

The House that Bob Built - in Januarythis year. Well, Hager Powertech haveproudly told us that they are themanufacturers of one of the unitsfeatured in the house.

Bob Symes, the Q.E.D. presenter,highlighted many of the benefits ofHager's Split Load Consumer Unitwith RCD protection. From thecompany's Cosmic and Invicta rangeof consumer units, the SLCU isdesigned for surface mounting, sosimplifying installation. Outgoingways are protected by miniaturecircuit breakers or modular fusecarriers incorporating BS 1361 HRCcartridge fuses. Any outgoing waysthat are not used for the initial

installation can be blanked offproviding spare ways for futureextension of electrical equipment.

The split load version is equippedwith a 100 amp double pole mainsswitch for isolating all circuits withinthe unit. The main bus bar system isdivided into two electrically separatesections. A maximum of six outgoingways can be controlled by the mainisolating device. An additional RCDis incorporated to control and provideearth leakage protection for a furtherfour or six outgoing ways.

Put safety first, ask for moredetails from Hager Powertech Ltd, 13Alexandra Way, Ashchurch BusinessCentre, Tewkesbury, Glos, GL208BNB. Tel : 0684 292901.

If you are organising any event to do withelectronics, big or small, drop us a line, weshall be glad to include it here.

Please note : Some events listed here may be trade or restrictedcategory only. Also, we cannot guarantee information accuracy, socheck details with the organisers before setting out.

May 30 -Jun 1. Computer Training and Services Show. Olympia.London. 01-486 1951.

Jun 26-28. Infrared Technology. Wembley Conference Centre. 079926699.

July 14-22. British Amateur Electronics Club (BAEC) exhibition.The Shelter, on the Esplanade, Penarth, South Glamorgan. This is a"hands-on" opportunity to find out more about the BAEC, don't missit!

Sep 18-21. EFTPOS 90. Exhibition and conference on electronicretailing. Alexandra Palace, London. 0273 722687.

Sep 25-27. British Laboratory Week. Olympia, London. 0799 26699.

Oct 2-4. Eurostat 90. Barbican Red Hall, London. 0799 26699.

Nov 6-8. Total Solutions. NEC Birmingham. 0799 26699.

NICAM GOESWEST

At the time of writing, Nicamdigital stereo should have come

to ITV and Channel 4 in the WestCountry and the North West duringMay as part of an IBA initiative tobring the new digital technology toalmost 80% of British viewers by theend of this year. South Wales was dueto start receiving the service from theend of April.

Nicam stands for Near -Instantaneously Companded AudioMultiplex, and we're glad the namehas been abbreviated! It adds aspecial digital signal to a standard tvtransmission to enable reception ofstereo sound with quality similar tocompact disc. The system can also beused to provide a second languagesound track. Stereo sound for tv ismuch welcomed for it adds anotherdimension to the whole viewingexperience, especially for music,drama, sport and light entertainment.Most recent feature films have beenproduced with a stereo sound track.

The number of stereo programmesand films on ITV and Channel 4 isincreasing. Those with digital stereosound are identified in TV Times, andon ITV's Oracle and Channel 4's 4 -Tel teletext services.

In order to receive Nicam, it isnecessary to have a tv set or videocassette recorder incorporating aNicam decoder. In the last year ortwo, a wide variety of receivers andvcrs have been produced with theoption of Nicam sound. Older setswith stereo speakers will probably notbe suitably equipped. The latter weremeant for connection to an externalstereo source, such as stereo vcrplaying back pre-recorded films.However, it may be possible to'retrofit' some of these older 'stereo'sets with a Nicam decoder, or makethe necessary connections to a Nicamvcr. Local dealers can advise you onthis. Once equipped with thehardware, no special aerialarrangements will normally beneeded, if the picture is acceptablethen the sound should be perfect.Nicam also offers a significantimprovement in mono sound.

PRACTICAL ELECTRONICS JULY 1990 5

N E-Ae-S

WIRELESSSOLDERING

Black and Decker are adding anew cordless soldering iron to

their Minicraft range. Minicraft is acomplete range of high quality,precision miniature tools for both diyand hobby applications.

The new cordless iron, modelMB650, is lightweight and has a builtin stand for ease of handling andextra convenience. Powered by a

screw in butane gas cartridge withsimple ignition, the iron enables theuser, say B&D, "to solder anywhere".(Bit of an over -broad statement isn'tit, B&D? I can think of many placeswhere you cannot, or should not, usea butane powered soldering iron.)

The price of the iron is £24.99including vat, and this includes twosoldering bits and a ready to usebutane cartridge, which gives aminimum of three hours continuoususe. Most hobby and craft shopsshould be stocking the iron.

NUCLEAREXCHANGE

With all the excitement relatingto the 'freeing' of Eastern

Europe, it comes as a bit of a partystopper to be told that some of BritishTelecom's clients continue to regardnuclear war as a continuing threatagairist which they should prepare. Ihave to sensibly admit that the currentfurore could just be a flash in the panand that the danger of some nutterpushing a nuclear button still exists,even though he/she may not be aRussian with anti -conservationisttendencies.

On those grounds perhaps weshould commend BT for theirforesight and success in winning amajor contract for digitalswitchboards which will continue towork after a nuclear explosion. Undera £5 million contract, BT will supplyMitel private branch exchanges(pbxs) to more than 300 localauthority emergency centresthroughout England and Wales, and afurther significant number mayfollow.

The contract demanded that thecompany met stringent technicalrequirements, and was awardedagainst competition from eight othercompanies. A requirement was thatthe equipment should be able towithstand the electromagnetic pulse(EMP) that follows a nuclearexplosion. EMP, say BT, would havea catastrophic result on anycomputerised equipment. We thinkthe computer operators would not betoo happy, either!

However, BT go on to say that

their successful implementation of thework will provide a modem, resilientnetwork which can providecommunications not only for waremergencies but also for peace -timedisasters.

BT'S ISDN2PLANS

British Telecom has announcedfurther plans for its ISDN

service, including tariffs and keydates for its introduction, which at thetime of writing was due to take placein April. The plans include newsupport facilities to assist terminalmanufacturers and customersdeveloping business applications.

From April, BT will be making alarger number of lines available formanufacturers to involve customersin their testing and developmentwork. This will give customers a tasteof the benefits ISDN can bring. Fromthe end of July the service will moveinto a market development phase.This phase will provide a largernumber of lines to customers inseveral business areas throughout thecountry. This will enable customers towork together with their terminalsuppliers and BT to establish pilotconfigurations to assess how best touse ISDN.

From January next year theservice will progressively becomeavailable throughout the country.Deployment to all digital localexchanges serving businesscommunities or high streets will beachieved by the end of 1991.

To find out more about ISDN,give BT a call. Your local exchangestaff will be pleased to help you.

CHIP COUNTSCC66470 VIDEO CONTROLLER

The SCC66470 from Philips is a new video and system controllerwhich can replace up to 30 ics by integrating all of the 'glue' logicnormally required for 68000 -based systems. This allows for theconstruction of colour display systems with a quarter of the numberof components required.

Featuring a resolution of 768 x 560 pixels, the new controllerwill appeal to a wide variety of system designers building pcs,colour monitors, videotex terminals, industrial man -machineinterfaces and CARIN process control systems. The ic works in CDI(interactive compact disc) systems and supports CDI format.

The chip is a colour display controller, system controller andimage manipulator in one ic. Only eight other ics are needed to forma complete display system. including keyboard controller.SCC68070 processor, 512Kbyte dram and 128Kbyte rom. Despiteits relatively high performance, the system is a fraction of the cost ofmulti -chip solutions.

CQL8OD VISIBLE WAVELENGTH LASER DIODES

Philips has. recently launched a new series of laser diodes whichemit visible light. The first type to be released is the CQL8OD whichemits red light at a typical wavelength of 675nm.

The diode is a gain -guided laser providing a maximumcontinuous wave output of 5mW. The anticipated lifetime isapproximately 250,000 hours at 3mW cw operation.

The laser is build on a ridge structure and is manufactured usinga MOVPE (moving organic vapour phase epitaxy) process on aGaAs substrate. It is mounted in a standard 9mm encapsulation. Amonitor diode which is optically coupled to the rear facet of thelaser controls the optical output level.

The low power consumption and simple supply requirementensure that the laser is suitable for portable hand held applications aswell as for replacing the conventional helium -neon laser in many ofits traditional uses.

74F5302 FIBRE -OPTIC LED DRIVER

The new fibre optic led driver 74F5300 and its dual counterpart74F5302 from Philips are ideally suited for high speed opticaltransmitter systems.

The driver's input buffer accepts ttl data. Its measuring circuitensures constant propagation delay and controls the rise and falltimes. In the dual type the propagation delays are matched. Theoutput driver is capable of sourcing or sinking more than 160mA atlow impedances.

When used with external pre -bias and pre -changing circuits theresponse can be tailored to a specific led resulting in an opticalwaveform with minimum duty cycle distortion.

These drivers are intended for use in areas such as high-speedserial data communication, fibre -optic data links, local andmetropolitan area networks, as well as digital and pbx systems.

SM DMOS FETS

All small signal vertical dmos transistors from Philips are nowbeing made available in the revolutionary SOT -223 surface mountpackage. This is the first surface mount type capable of dissipatingI W on a standard printed circuit board.

This move combines the advantages of the fast switching speedof dmos (turn -on times as low as 4ns) with the benefits provided bysurface mounting in terms of ease of assembly and cost/spacesavings.

Principal applications for surface mounted dmos fets includetelephony, as line current interrupters in the ringer and flash loopbreak circuit, as well as electronic hook switch in the hold circuit,and in other applications where low or high voltage switching isrequired, such as EDP, electronic ballasts, small motors and relays.

MANUFACTURER'S ADDRESSPhilips Components Ltd, Mullard House, Torrington Place,

London WC1E 7HD. Tel : 071-580 6633.


CONFACTINSC PLAYER CD840

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BITSTREAM ON STREAMPhilips tell us that their excitingnew development, Bitstream, has

arrived with the recent official launchof their CD840 cd player.

The CD840 is the first of Philip'scd players to feature the company'srevolutionary one -bit digital toanalogue conversion system whichoffers 256 times oversampling. It isclaimed to be the most significantimprovement in sound reproductionsince cd was first introduced.

This new advanced model alsoboasts a whole host of innovative anduser-friendly features. FavouriteTrack Selection (FT'S), an exclusive

facility on the top -of the range Philipscd players, has been developed toform new Double FTS. Thisextremely convenient feature has twoFTS memories to cater for separaterequirements, such as pop andclassical pieces, or personal choicetracks and party music.

Another feature, FTS Info,compiles and updates the Double FTSmemories summarising their statusthrough display messages whichinclude remaining memory space andtitle mode. Favourite tracks can beeasily identified by words of up to 12characters once they have been

programmed into the unique TitleMemory.

Other features include full infra-red remote control, selectable 10 or20 second music scan, 3 -speedselectable forward/backward searchand random order programming.

An amazing feature of this cd isthat it will draw your attention to it inyour local high street store: it has aninnovative Trade Mode, designed tocatch the attention of potentialcustomers. In this mode it uses abuilt-in programme to promote itselfin-store by automatically displayingactive info messages as it runs

through the players main features.While in progress the disc is lockedinto the machine; to prevent theft, sayPhilips!

The Philips CD840 has digitaloutput, not only through a standardhigh grade electrical connector withgold plated low -noise contacts, butalso through a standard opticaloutputs.

This superbly styled, highperformance player is targeted firmlyat the audiophile, and is currentlyavailable at a retail price of £349.99.

RUBBERDUCKING

If you're looking for something toprotect your favourite bit electronicgadgetry from water, Rubbaweldcould possible provide the answer.

Rubbaweld is a self -amalgamatingtape that welds to itself on contact.

SEER -AMALGAMATINGWATERPROOF R.I.Y. TAPE

AV Wei 141

'1:70The we Ilsol iriN16 Noll.*solid wino/proof Wyk

It's applied by stretching andwrapping the tape around the objectto be sealed. The tape then forms asolid rubber coating which providesexcellent electrical insulationproperties and protection againstwater penetration. It protectselectrical fittings from water andchaffing, making it ideal for outsideaerials and other external electrics.Other applications include generalelectrical work, as well as hose andpipe connections and repair.

The tape is supplied in a 3m x25mm roll, complete withinstructions, for only 2.65, includingvat, post and packing.

Sounds like excellent material foranyone with a waterproofingproblem. Perhaps you can now eveninstall electronic remote control inyour bathroom rubber duck.Whatever your application, contactGeedon Performance Coatings,Commerce Park, Whitehall Road,Colchester, Essex, CO2 8HX. Tel :0206 47556.

EASYMEASURE

The latest handheld digitalmultimeter from TMK provides theuser with all the basic measurementsusing a single, easy to use, rotaryswitch. Model G85 has a clear,unambiguous 1999 count, 3.5 digitliquid crystal display with automaticzero, polarity, overrange and lowbattery indication.

Housed in a yellow rugged case,the meter has recessed safety socketsand test leads. (Why are all metersseemingly produced in yellow thesedays? Surely there are other coloursin the rainbow to choose from?)

Two dc voltage ranges measure to200V, have a 10mV resolution and abasic accuracy of 0.7%. AC voltagegoes to 500V in one range with a IVresolution. Direct current is to 10Awith a 10mA resolution and a basicaccuracy of 1.5%. Ohms aremeasured in two ranges to 2M ohms,

ADVERTISERS AWAKE!Don't let your competitors steal allthe glory of PE News Page

Publicity - have YOUR interestingnew products highlighted here as wellas theirs! Send us concise details plus

a good photo and we'll do our best topublicise them

First come first served, and it mustbe interesting. It's up to you to keepus all informed!

with a best resolution of 1 ohm.Diode tests are carried out at 3V and1 mA.

Model G85 is supplied ready foruse with a battery, test leads, carryingcase and instructions. Its size is 73 x148 x 32mm, and it's fully guaranteedfor one year. The price, excluding vat,is 39.95.

For further information contactMike Dixon, TMK Instruments,Building 3, GEC Estate, East Lane,Wembley, Middx. HA9, 7PJ. Tel:081-908 3355.

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T he cellular phone operators, Cellnetand Vodafone, remain seeminglyunconcerned about the problem of

eavesdropping by people with scanners. Moreaccurately, they seem unconcerned about theway the popular press now paintseavesdropping as a hobby. At Easter my pre -Christmas calls to Cellnet's new head man,Stafford Taylor, still remained unretumed. Ohwell, he did once work for IBM.

There is no way to stop eavesdropping. Itis not illegal. It is seldom harmful. But therisks are obvious and the law is intended tostop the popular press glorifying it and tradecapitalising from it. But if Cellnet andVodafone made too much fuss they mightdeter people from subscribing to theirservices. Education loses customers.

This is exactly why it has taken over sevenyears for the public to wake up to the fact thatdomestic cordless telephones are appallinglyinsecure. If the manufacturers had been frank,sales would have been far slower.

HORROR STORY

appears at a lower frequency than the original.The image is twice the intermediate frequency,or IF, of the radio receiver away from the radiosignal; and for a medium -wave radio the IF is at470 kHz. So the image of the 1.6 MHz cordlessphone signal is slap inside the mw band.

Some radios are more likely to pick up theimage signal than others. It all depends on their"image rejection", which in turn depends on theability of the circuitry at the "front end" of thereceiver to reject all frequencies from outside themw band. And of course quality depends on cost.

Modern budget radios, especially personalstereos, have poor image rejection. Hence theyare very good at picking up cordless telephonecalls. The saving grace here is that thetransmission power of a cordless phone islimited to 10 milliwatts, which is very weak. Sopickup usually only happens when the phoneand radio are in the same house, or separated bya flimsy party wall.

But, as proved by the above horror story, ifthe cordless phone is being used at the top of ahouse, on high ground, and if the user has strungout the wire aerial that comes with the phone, itssignal may travel literally miles.

INSECURE CELLSFirst, let me recount a horror story.Recently a friend of mine heard a knock at

the door. There stood a complete stranger whoknew all about his life - the number of hisbank account and credit cards, the names ofhis friends and relations and, most worryingof all, the time when he was due to go intohospital and leave his home unattended.

My friend was lucky. The call wasfriendly. The visitor, who lived at least a mileaway, had for months been picking up callsfrom his cordless telephone on a mediumwave radio. He got the address from atelephoned delivery order and came roundwith the warning - "watch what you say".

I called round, and we tried several radiosin the house and out in the street. All pickedup signals, but some far better than others. Atfirst we thought it was a fault on the cordlesstelephone which, by the way, is a fullyapproved "green circle" model widely onsale. Foolishly, I had forgotten the faultinherent in the existing (CT1) cordlesstelephone standard set by the DTI.

LEGAL FREQUENCIES

Originally, all cordless phones were illegalin the UK. Then, in August 1982, the DTIallocated eight radio channels, each 20kHzwide. To control imports, the approvedfrequencies were deliberately slightlydifferent from those used in the US, around1.6MHz from base station to handset and47.45-47.55 MHz from handset to basestation (instead of 49.8-49.9MHz for the US).Cordless phones operating on thesefrequencies became legal on 1st January1983.

BY BARRY FOXWinner of the

UK Technology Award

It's not necessarily BigBrother who may be

listening to you, it couldbe all your neighbours,friends or foes, on their

budget trannies.It is the choice of frequencies at around 1.6

MHz which causes the trouble. The 20 kHz bandchannels start at the centre frequency 1.642MHz(Channel 1) and go up to 1.782MHz Channel 8).Although medium wave radio tuners are usuallyonly marked up to 1.602 MHz, there is often agrey area which catches the lower cordlesschannels.

Although the offending transmissions arefrom the base unit to the hand -set, the phonesystem mixes both halves of the conversation, soboth are heard. The obvious solution is for shopsto sell only cordless phones that operate on thehigher channels. But, then there is a higher riskof complaints from two neighbours hearing eachother's calls.

IMAGING

Even if people do not tune past RadioLuxembourg and thus never stumble on thecordless phone leakage zone that follows, thereis still a risk of medium -wave breakthrough.Any modern superhet radio has the unfortunateability to pick up the "image" of a signal which

AWARENESS

The most sobering aspect of all this is not somuch the DTI allocated frequencies that havecaused the problem, but that the makers ofcordless telephones and engineers who test themare all fully aware of the situation. In the eightyears since cordless phones went on sale in theUK, no-one seems to have made any effort towarn the public that their personal calls are likelyto end up being heard on someone else'smedium -wave radio.

When Radio 4 recently reported on howScottish businesses had discovered that theirconversations were being overheard by peoplewith medium -wave radios tuned to the far end ofthe dial, past Radio Luxembourg, there was thena flim-flam response from BT about it being onlya "minor problem" with "few complaints".

Of course there have only been a fewcomplaints, because the phone owners whosecalls are being overheard, seldom know it.

On the face of things, the simple solution is tobuy one of the new "second generation" or CT2cordless phones. Although the transmissionpower is the same 910 mW), the frequencies aredifferent and the speech is in digital code, makingaccidental eavesdropping virtually impossible.The snag here is that CT2 has been sold primarilyas a handset -to -public base stations system, andkilled by the mish mash of incompatiblestandards which make that use impractical. If itwere sold as a handset -to -home base station itmight succeed, provided that the kit cost can bereduced from current levels of several hundredpounds.

Tom Ivall looks at celiphones from a differentangle on page 57. We shall be discussing theirtechnicalities in greater depth next month. Ed.


T hree faces of technological conser-vation have come to my attentionrecently. Only two of them reflect a

positive attitude.On the positive side, SRIS, the Science

Reference and Information Service of TheBritish Library has announced the introduc-tion of its new Environmental InformationService. The intention of the service is to

help industry solve the problems of waste,pollution and the quality of our surroundings.

SRIS comments that green enthusiastsare to be seen everywhere in business, notonly because of a wish to be 'good corpo-rate citizens', but because it is becomingincreasingly difficult to avoid the penalties offalling sales, obsolete products and prose-cution. In collaboration with other relatedinformation sources, the SRIS is in a uniqueposition to support managers in the environ-mental control of their products. The Envi-ronmental Information Service has beendesigned to bring the benefits of 'one -stop -shopping', so that a one call to a singleenquiry point will give immediate access toall British Library resources.

Conservation is not just about protectingthe environment, it's about protecting ourheritage as well. This includes preservingitems that have played a significant part inour history so that future generations maybenefit from direct knowledge about them.Thus I welcome the second positiveannouncement.

It has come from an organisation devot-ed to preserving technological history, theDuxford Airfield Radio Society. Associatedwith the Imperial War Museum and based

PRACTICAL

ELECTRONICS

near Cambridge, the Society's aims are todocument, collate, restore and occasionallyoperate historic military radios, mainly ofBritish origin. The equipment has come fromthe collections of the members as well asfrom the IWM. Among the collections areSpecial Forces and clandestine radios, andcommunications equipment. Regular exhibi-tions relating to particular themes are heldperiodically. The Society's efforts are warm-ly applauded.

I do not applaud, though, Commodore'sinability to help me with another aspect ofconservation. I have a ten-year old

Commodore 3032 (PET) computer. It hasbeen an extremely reliable machine, robustand easy to use. For several years its main

function has been as a piece of electronicworkshop test equipment, controlling andmonitoring experimental digital designs.Recently, a minor screen fault developedwhich I started to repair, but carelesslyshorted out the eht. The screen is nowdead.

Not having a circuit diagram for this partof the computer I know neither the crt pinconnections, nor those of the eht coil, leastof all do I know the required tube voltages. Irang Commodore, who said I should contacttheir agents. The agents stated that they donot have information for a machine "thatold". Back to Commodore, and through aseries of different departments. But none ofthem could supply the required data sheets,nor could several companies who claim tohave extensive ranges of ancient and mod-ern data sheets and manuals.

It is with great concern that I regard theapparent inability of at least one manufac-turer to supply servicing information forequipment that is not excessively old. The3032 may be obsolete with regard to thecapabilities of current technology, but by thestandards of its day it performed excellentlyand I have no wish to junk it just because Ican't get a data sheet. It's ironic that Dux -ford have working items of equipment over50 years old, yet here is an item one fifththat age and seemingly not intended tobecome a working example of past history.Manufacturers and their agents shouldensure that data for their past modelsremains on file for the benefit of those whoknow how to use it.

THE EDITOR

Editor:John Becker

Sub -Editor:Helen Armstrong

Technical Illustrator:Derek Gooding

Advertisement Executive:David Bonner

Production Manager:David Hewett

Office Manager:Tim Lyster

Publisher:Angelo Zgorelec

Practical Electronics,Intra House, 193 Uxbridge Road,

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Readers' EnquiriesAll editorial correspondence should be

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We regret that lengthy technical enquiriescannot be answered over the phone.

AdvertisementsThe Publishers of PE take reasonable

precautions to ensure that advertisementspublished in the magazine are genuine, butwe cannot take any responsibility in respectof statements or claims made byadvertisers. The Publishers also cannotaccept any liability in respect of goods notbeing delivered or not working properly.

Intra Press 1990. Copyright in all drawings, photographsand articles published in PRACTICAL ELECTRONICS is fully

protected, and reproduction or imitations in whole or in partare expressly forbidden. All reasonable precautions aretaken by PRACTICAL ELECTRONICS to ensure that theadvice and data given to readers is reliable. We cannot,however, guarantee it, and we cannot accept legalresponsibility for it. Prices quoted are those current as wego to press. All material is accepted for publication on theexpress understanding that the contributor has the authorityto permit us to do so. ISSN 0032-6372

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CRYSTALS2.771A -1z/4.000 MHz/4.9152MHz 20MHz 49.504MHz, MA, 16.588M ....Li each

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14 WAY EF SKT ........ .......... ............................ ...£1

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02.50 100+ 01.50HALL EFFECT IC UGS3040 + MAGNET £1

4C71c8:CHEAP PHONO PLUGS 100/C2 1000/0161

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1 pole 12 way rotary switchAUDIO ICS LM380 LM386 ea LM324 4t1555 TIMER 5/C1 741 OP AMP 5/CIZN414 AM RADIO CHIP

48/C4)P1COAX PLUGS nice onesCOAX BACK TO BACK JOINERS 3/C14 x 4 MEMBRANE KEYBOARD 01.5015.000uF 40V C2.80 (C1.25)INDUCTOR 20uH 1.5A 5/CINEW BT PLUG + LEAD 01.501.25" PANEL FUSEHOLDERS 3/CICHROMED STEEL HINGES 14.5 x 1" OPEN C1 each12v 1.2w small wire ended lamps fit AUDI VW TR7 SAABVOLVO12V MES LAMPS 10/C1STEREO CASSETTE HEAD C2MONO CASS.HEAD Cl ERASE HEAD 50pTHERMAL CUT OUTS 50 77 85 120"C C1 asTHERMAL FUSE 121°C 240V 15A 5/C1TRANSISTOR MOUNTING PADS TO -5/70-18 C3/1000TO -3 TRANSISTOR COVERS 10/CISTICK ON CABINET FEET 30/C1

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DIODES AND RECTIFIERS1N4148 100/C1.501N4004/SD4 1A 300V 100/C31N5401 3A 100V 10/CIBA158 1A 400V fast recovery 100/C3BA159 lA 1000V fast recovery 100/C4120V 35A STUD 65pBY127 1200V 1.2A 10/C1

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WIRE WOUND RESISTORSW21 or sim 2.5W 10 of one value C1

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STC NTC BEAD THERMISTORSG22 220R, G13 1K, G23 2K, G24 20K, G54 50K, G25 200K, RES@ 20°C DIRECTLY HEATED TYPE £1

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IC SOCKETS6 pin 15/CI 8 pin 12/CI 14/16 pin 10/CI 18/20 pin 7/C1,22/24/28 pin 4/C1 40 pin 30p

SOLID STATE RELAYS40A 250V AC SOLID STATE RELAYS C15

POLYESTER/POLYCARB CAPS100n, 220n, 133y 5mm 20/£1 100/£31n/3n3/5n6/8n2/10n 1% 63v 10mm 100/EIS10n/15n/22n/33n/47n/68n 10mm rad 100/C3.50100n 250v radial 10mm 100/C3100n 600v Sprague axial 10/C1 100/C6 (C11

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Anyone who believes that, in these daysof extremely hi -tech communications,the age of morse code is dead, is

thoroughly mistaken! Just take a listen on areasonable radio receiver with a good aerial.Scan the tuning scale across the frequencybands and you'll find plenty of examples ofpeople communicating around the globe usingmorse.

Morse has the great benefit of being able tobe received and understood under atmosphericconditions that would make speechunintelligible. This makes it an ideal method forcommunicating over large distances or withrelatively low -powered and simple equipment.Proficiency in morse code is still a pre -requisitefor anyone wishing to hold a Class 'A' RadioAmateur's Certificate.

The morse decoder to be described here willbe of interest to both aspiring radio amateursand inquisitive radio listeners. For the formerthe unit can be used as a valuable piece ofequipment for checking their code -keyingaccuracy. For the latter, it offers a means bywhich seemingly unintelligible di-di-dah-dah-di-dits can be understood without the need tolearn morse code.

MORSE DECODERDISPLAY MODULE

The heart of the unit is an intelligent liquidcrystal display module. This has a library ofpre-programmed alphanumeric and punctuationsymbols which can be displayed on the lcdscreen in response to specific codes. Basically,the unit receives the morse code and converts itinto ,the correct library address number for theequivalent symbol. The decoded message flowsacross the eight -symbol lcd screen from right toleft at the speed with which it is received. Allthe user needs to do is adjust three panelcontrols to match the transmission rates for thedots, dashes and pauses.

The lcd module used is the Sanyo LCM570,which is also available as the Electromail/RS585-006. It was described in detail in PE Mayand June 90. Readers who missed the articlescan obtain copies at the price quoted on theEditorial page. You don't, however, need thoseissues in order to build this morse decoder.

The symbol library within the lcd modulecontains a complete set of alpha characters fromA -Z, the numerals 0-9, and punctuation marks.The same symbols are represented in morsecode, but the codes relating to them aredifferent. We need, therefore, to translate themorse code into the code which will cause thelcd module to show the correct symbol on thescreen. This is carried out in the decoder byusing an eprom or eeprom as the pre-programmed conversion device.

CONVERSIONCONCEPTS

John Beckerdescribes a semi-intelligent aid forinquisitive radio

listeners andamateur radio

students.ALPHABETA . -B -C -h -E .

F . - -H .

I .

K -L . -M - -N -t]--P --R

T -U . - . -W - . -Y - -

HUNERALE;

4 . . .

- . .

PUNCTUAT I ON

- --

by the decoder as a binary number relating to Table 1: Morse codesEach morse code character is interpreted

the dots and dashes that go to make it up. Thisnumber is then regarded by the eprom as theaddress at which another number is stored.The stored number is that which the lcdmodule recognises as the code for thecharacter we want to see displayed.

For example, the lcd address code numberfor character 'C' is 67. In morse code, letter`C' is represented by the code '---' (dash dotdash dot - usually pronounced dah-di-dah-di).If we regard the dots as 'zeros' and the dashesas 'ones', letter 'C' can be re -coded as '1010'.This, of course, can now be treated as a binarycode, the equivalent decimal number of whichis ten. If, then, we program the eprom to holddecimal 67 at its address decimal 10, theneach time the morse code converter sendsdecimal 10 to the eprom, the eprom will placedecimal 67 at its outputs. Suitably triggered,the lcd will then show letter 'C' on its screen.A similar principle can be used for eachcharacter in the morse code.

BIT PROBLEMIf you look at the morse code table in

Table 1 you will see that a potential problemexists regarding the conversion of somecodes. Take letters E, I, S and H, for example.E is , I = , S = and H= . Translatingthese codes to binary produces 0, 00, 000,0000, all of which codes are similarrepresentations of zero. A similar problemarises with T, A, U and V, which all have oneor more dots before the final dash, convertingfrom -, , , -- to 1, 01, 001, 0001. It isobviously only the final dash, or I, which hasany significance.

There is an electronic factor to be takeninto account as well. I use a 6 -bit serial to


.1461.COMMUNICATIONS PROJECT

parallel shift register to clock in each of thezeros and ones, and, in effect, it reverses theorder of the bits. (The register is actually an8 -bit one with the final two bits ignored). Theshift register is first of all set to zero, socontaining binary 000000. If T is received, asingle 1 is clocked in from the left of theregister, resulting in the contents becoming100000. If A (.- = 01) is being received, thedot is first clocked in as a zero, so the registerstill holds 000000, then the dash is clocked inas a one, so the register now holds 100000,the same as for T. The same 100000 resultswith U and V. In this order of reading, each ofthese four letters therefore produces a decimaladdress of 32. Similarly, E, 1, S and H willproduce no apparent change in the contents ofthe shift register, all resulting in decimal 0.

To overcome the problem, we need to takeanother factor into account: the total number ofdots or dashes that are contained in the

particular morse code character. For the sakeof this secondary conversion we ignore thedifferentiation between the dots and dashes,treating them simply as a single bits of data. Inthis way E has one data bit, so too does T, I hastwo data bits, as do A, I, M and N. If you lookdown the code list you will see that themaximum number of bits shown for any codeis six. If we take into account the total numberof bits received for a particular character, aswell as the order of dots and dashes that go tomake it up, then each individual code can beconverted into its own specific numericaladdress equivalent.

By treating the bit count of each code as abinary number and attaching this to the righthand side of the binary conversion of the dotsand dashes, you will see how this works. Threebinary bits can represent all decimal numbersbetween 1 and 6, whose binary codes are thus001, 010, 011, 100, 101 and 110, respectively.

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Let's attach these codes to the right handside of the shift register's 6 -bit contentsfollowing the receipt of any of T, A, U or V,and convert the new code to decimal:

T (-): register (100000) + count (001) =100000001 = 257.

A ( -): register (100000) + count (010) =100000010 = 258.

U (-- -): register (100000) + count (011) =100000011 = 259.

V ( -): register (100000) + count (100) =100000100 = 260.

Similarly, E, I, S and H will produce codesof decimal 1, 2, 3 and 4, respectively. The fulllist of converted addresses is given in Table 2.

There are a couple of points you willprobably spot in the table. Firstly, the samecode is used in morse for both the colon andsemi -colon; the unit has been programmed todisplay the semi -colon on receipt of this code.

Secondly, it would appear that some lettersare repeated but with different addressnumbers. This is to allow for alternativeversions of the letters A, E, N, 0 and U thatmay be used by some Continental operators.The unit has been programmed to display theEnglish equivalent letters for these codes. It

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has also been programmed to display ahighlighted blank (lcd code 255) for anyunrecognised code received.

Although the maximum bit count of anymorse code has been taken as six, there areexceptions to this. For example, there is an 8 -bit code of ' which is transmitted tomean "error". The unit does not allow for thiscode. Nor does it allow for some other codeswhich have specific word or phrase meanings,eg, = "long break", and - =

"understood".As I said earlier, each of the individual

Table 2. Conversion table.

A 100000010 258 65B 000100100 036 66C - . 010100100 164 67D 001000011 067 68E 000000001 001 69F . . 010000100 132 70

011000011 195 71H 000000100 004 72I 000000010 002 73

111000100 452 74101000011 :323 75

L 001000100 068 76110000010 386 77

H 010000010 130 78U 111000011 451 79P 011000100 196 80Q 101100100 356 81R 010000011 131 82

000000011 003 83T 100000001 257 84

100000011 259 85100000100 260 86

IJ . - 110000011 387 87100100100 292 88110100100 420 89001100100 100 90

0 111110101 501 481 111100101 485 49

111000101 453 50110000101 389 51

:# 100000101 261 52000000101 005 53

6 000010101 021 54000110101 053 55001110101 117 56

9 011110101 245 57- 101100110 358 33010010110 150 34011110110 246 39011010101 213 40101101110 366 41010100101 165 43

- 110011110 414 44- 101_1001110 270 45

. - - 101010110 342 46010010101 149 47000111110 062 59000111110 062 59100010101 277 61001100110 102 63101000100 324 97101100101 357 97

E - . . 001000101 069 101- - 110110101 437 110011100100 228 111

u - - 110000100 388 117

address codes produced by the shift registerscauses the eprom to produce another codenumber on its output. This numbercorresponds to the code required by the lcdmodule in order to display the equivalentsymbol or character. The eprom has also beenprogrammed to perform two other functions.Firstly, at switch on, the eprom transmits an'initialisation' routine to the lcd module inorder to set it into the correct control mode.

Secondly, I have designed in the option foradditionally displaying the actual dots anddashes that make up the code being received.This option can be switched in and out at will.A third option designed in, though notcontrolled by the eprom, is the ability toautomatically have blank spaces displayedbetween complete words. This may beswitched on or off as required.

TRANSMISSION FACTSThere is a wide variety of rates at which

morse code operators can and will transmit.The main requirement fora skilled operator isthat his or her transmission rate should beconsistent and conform to relative lengths ofdots, dashes and pauses.

Transmission rates are usually expressed interms of the number of words sent per minute.Conventionally, in this context one word isregarded as having a length of five characters.Good operators can send or receive at rates of25 or more words per minute. Irrespective ofthe transmission rate, internationalconventions dictate that operators shouldideally make their dashes three times thelength of their dots. The spaces between eachdot or dash should be the equivalent length ofone dot. The pauses between individual lettersshould be the equivalent of three dots, and thepauses between words should be equal inlength to seven dots.

Because operators will vary from eachother in their transmission rates, and in theprecise timing of dots, dashes and pauses, anyautomatic morse decoder has to be designedwith a wide degree of in-built flexibility. Witha sophisticated computer -controlled decoder,the software can be written to make its ownassessments and corrections in order to lockon to any skilled morse operator'stransmission. Indeed, some years ago I wrotea machine code routine for a 6502 -basedcomputer that did just that, taking the signaldirectly from a radio receiver via a simpleamplitude controlling interface.

The unit being described here, though, hasbeen designed as a simple decoder in whichpanel controls are used to preset the relativerates for the dots, dashes and pauses. The'intelligence' of the unit then comes into playin the automatic interpretation of the receivedcodes.

The basic functions to be designed for are:detection of the signal pulses, separation ofdots from dashes, separation of letter pausesfrom word pauses, formatting of the receivedcode for visual display. Fig.1 shows thecomplete circuit diagram of the morsedecoder.

PULSE DETECTION

Signal modulation detection is carried outin a fashion closely resembling that which Iused in the PE Radio Clock (March -June 90).A phase locked loop, ICI, is fed with theaudio signal containing the morse codemodulation, ie, the frequency pulses. Thesignal can come from any source, such as aradio, microphone, cassette or tape recorder,morse key, or computer, etc. The mainprovisos are that the pulse amplitude shouldbe greater than 500mV, but no more than 5V,and that the signal should be relatively free ofbackground noise.

The extracted pulses go to the phasecomparator input of IC1 at pin 14. The chiphas a second phase comparator input at pin 3into which is fed a frequency from an internalvoltage controlled oscillator, which has itsoutput at pin 4. Internal circuitry detects thedifference in phase between the two signalsand the voltage level present at pin 13 reflectsthat difference. Using R5 and C3 to smoothout minor bits of noise fluctuation, the signalis fed from pin 13 to pin 9, which is the vcoinput. The vco then attempts to vary itsfrequency so that there is no phase differencebetween it and the external signal. Since it's apulsed frequency that is being input to IC1 pin14, the resulting voltage output at pin 13 willrise and fall in sympathy with the pulses. Inthe presence of a pulse the output at pin 13will be high, and when the pulse ends theoutput will revert low. The actual amplitudeswing will depend on the strength of the inputpulse. Under ideal strength and signal clarityconditions the swing should be close tomaximum, ie, between +5V and OV. Tomaximise this condition, the values of C2, R4and R5, which control the vco primaryfrequency, have been chosen so that zerophase difference can never actually bemaintained between the two signals.

The pulsed signal at ICI pin 9 is taken tothe inverting buffer IC2a. Its output is theninverted by IC2b, so maintaining the samesignal phase. The output from IC2b is routedin four directions, to a pulse length extractor,two pause extractors, and to the first of twoshift registers.

DOT -DASHEXTRACTION

The pulse length extractor is formedaround IC2c-e, and it is this stage whichdetermines whether the pulse is a dot or adash. When a morse pulse is detected by ICI,the resulting effect is that the output of IC2bgoes high. This causes the NAND gate IC2cto trigger its output low, causing a negative -going pulse across C4. In response, IC2d'soutput changes from low to high, feedingback to the second input of IC2c. Furtherlevel changes from IC2b are thus preventedfrom having any more affect on IC2c until theoutput of IC2d reverts low again. This willonly occur once C4 has recharged via R6 andVR1 to a level above IC2d's trigger point, inother words, IC2c/d have been configured as a

14 PRACTICAL ELECTYRONICS JULY 1990

ICOMMUNICATIONS PROJECT TIT

non-retriggerable monostable. If you care tocheck, this is exactly the same technique asused in the Radio Clock.

4C2d's output is inverted by IC2e andbecomes the clock signal for the two shiftregisters IC3a and IC3b. You will havespotted that the pulse length generated by themonostable IC2c/d is independent of the pulselength from IC 1. This means that at the end ofthe monostable's period we can detectwhether the output pulse from IC1 is stillhigh, or whether it has by this time revertedlow. Thus we can determine whether ICI 'spulse can be regarded as a 1 or a 0, as a dashor a dot.

The comparison is automatically carriedout by the shift register IC3a. The `morse'pulse output from IC2b is used as the datasignal for the shift register. When the registeris triggered by the high -going output fromIC2e, the register will clock -in the levelpresent on its data input. So, if the data inputis high, a 1 will be clocked in, but if it's

COMPONENTSRESISTORSR1, R2,R5 -R7, R10,R11, R14,R17, R18 100k (10 off)R3 47kR4, R8 1M (2 off)R9, R13, R16 10k (3 off)R12 510kR15 1kAll 0.25W 5% carbon film

CAPACITORSC1 -C3, C5,C8, C9, C12,C16, C18,C20 -C22C4C6;C7, C11,C13, C15C10, C19C14C17C23

100n polyester (12 off)220n polyester

1µ 16V electrolytic (5 off)22n polyester (2 off)4µ7 16V electrolytic1n8 polystyrene470µ 16V electrolytic(optional - see text)

SEMICONDUCTORSD1 -D8 1N4148 (8 off)[Cl 4046IC2 4572IC3 4015IC4 4520IC5 4584IC6 74HC157IC7 eprom or eeprom (2048 x 8

bits - see text)IC8 7805 (optional - see text)

POTENTIOMETERSVR1-VR3 1M lin rotary (3 off)VR4 10k log rotary (optional - see

text)VR5 100k skeleton preset

SWITCHESSi min dpdt toggleS2 morse key (see text)

MISCELLANEOUSLcd module type LCM570 (see text).Dil ic sockets : 14 -pin, 16 -pin (5 off), 24 -pin.Printed circuit board, pcb supports (4 off),knobs (3 or 4 off - see text), 3.5mm jacksockets (2 off), box to suit (approx 180 x120 x 40mm).

already reverted low, then a 0 will beregistered. A similar process will occur eachtime the register is clocked, shifting all thepreceding data through by one place.

The fourth output of the first register isconnected to the data input of the second,IC3b, which is clocked by the same signalfrom IC2e. Between them, IC3a/b have eightoutputs (though only six are used) which holdthe current parallel equivalent of all the serialdata bits clocked in. The outputs areconstantly open, putting their data onto theaddress lines of the eprom, IC7. The epromthen places upon its own outputs the data heldin memory for that particular address, sendingit to the lcd module. The module, though,won't respond to it until its Enable line issuitably triggered.

LETTER PAUSESWe constantly need the circuit to check for

pauses between groups of pulses so that theend of each transmitted character can bedetected. It is only at that moment that wewant the lcd module to update its display.

The letter -pause detector is formed aroundIC5a. This is another monostable, but onewhich can be retriggered before its previouspulse has ended. The high -going pulse fromIC2b almost instantaneously charges C7 viaDI, causing IC5a, which is an invertingSchmitt trigger, to be triggered into changingits output state from high to low. Thisoccurrence causes C9 to discharge via D3, sotaking (indirectly, via the gate IC6 - more ofwhich presently) the reset pins of the shift

registers low, allowing them to accept datafrom that moment onwards, until the resetpins are taken high again.

When IC2b's output falls low, at theending of the morse pulse, C7 begins todischarge via VR2 and R9. If C7's level hasnot fallen below the Schmitt threshold levelby the time the next morse pulse arrives, C7will immediately recharge, and there will beno change in state of IC5a. If, however,another morse pulse fails to arrive before C7has discharged sufficiently, then IC5a's outputwill revert high once the threshold has beencrossed. When this happens, a short positivegoing pulse is generated across C8, CIO andR10, the negative going edge of which (viaIC6) triggers the lcd module into acceptingthe data from the eprom. In so doing, itupdates its screen display with the symbolcorresponding to that data.

The high level on IC5a now allows C9 tocharge up via R11. Once C9 has chargedsufficiently, the reset pins of the shift registersin IC3 will go high, so resetting the storeddata back to zero. The registers are now readyto start building up a new sequence of data forthe next letter code transmitted.

WORD PAUSESThere is a third monostable, around IC5b.

This is the one which is responsible fordetecting pauses between words. It works onthe same principle as that around IC5a, andhas its output triggered low by the samepositive going pulse from IC2, via D2 feedinginto C11. The negative going output from

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IC5b has no effect upon any subsequentcircuitry. It's only the positive -goingtransition that is relevant. The monostable'stiming factor is set for a period longer thanthat for IC5a. When the morse pulse fromIC2b ends, C11 begins to discharge via R12and VR3, and once the Schmitt threshold hasbeen crossed, IC5b's output reverts high. Apositive going pulse is generated across C18,C19 and R17, and passes via D5 to the lcdmodule's E line. Since the monostable aroundIC5a has already reset the shift registers bythis time, the module is triggered to accept thedata at eprom address zero, which is pre-programmed to hold the code for a blankspace. Consequently, the lcd screen now hasthis blank space placed immediately after theprevious letter being displayed.

BIT COUNTINGSo far we have established how the dots

are separated from the dashes, and how theletter and word pauses are detected. The nextthing to look at is the counting of the bits thatmake up each letter.

This is very straightforward, and simplyrequires the use of a pulse counter. IC4 is usedfor this purpose and it is a 4 -bit (0-15) binarycounter of which the first three outputs areprimarily used for letter -bit counting. They

Fig 3. PCB component layout.

are also, together with the fourth output, usedin the switch -on initialisation process, as weshall see shortly.

The counter's clock input is fed from theoutput of IC2e (via the gate IC6) and so istriggered each time the monostable IC2c/d istriggered by a morse pulse, whatever itslength. The counter's outputs then become theextra three address data lines required by theeprom to complete the necessary nine digitcode. Resetting of the counter occurs at thesame time that the registers are reset. Thesame line from monostable IC5a is used,feeding to IC4a via C5 and R7. The lattergenerate a short high -going reset pulse,allowing the counter to count even when theregisters continue to be held reset. This factoris important to the switch -on initialisationroutine.

DISPLAY MODESThere are two modes in which the morse

data can be displayed. In the first, only thecharacters and pauses are displayed on thescreen. In the second mode, the samecharacters and pauses are displayed, but theyare preceded by the display of the actual dotsand dashes that make up the characters. S Iswitches between the two modes.

In the characters -only mode both poles of

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In order to have the dots and dashesdisplayed, S lb is switched to bring the outputof IC5e into circuit with eprom address lineA10, and S la switches the monostable IC2c/donto the E line bus.

As we saw earlier, when the first morsepulse arrives, the monostable around IC5a istriggered so that its output goes low. Thislevel is inverted by IC5e and sets the eprom toa different set of address locations, asdetermined by line A10. The eprom willremain with A10 high until the letter pause isdetected by the monostable. With A10 high,the eprom addresses controlled by the shiftregisters have been programmed to place thelcd codes for dots and dashes at the eprom'soutput. If the shift register IC3a clocks in azero from its data input, then the lcd dot codeis produced, and of course, a one produces thedash code.

The lcd module is triggered to display therelevant dot or dash each time it occursbecause the E line is simultaneously toggledvia D6 by the monostable IC2c/d.

As soon as the letter pause occurs, addressline A10 reverts low, so the eprom reverts toits normal set of addresses, and displays thereceived letter in the normal way,immediately after the last dot or dash.

LCD SUMMARYAs readers who read PE May and June 90

will know, the lcd module is an extremelyversatile device. For those who missed theprevious two articles, I'll just quickly recapon some main points.

The module can be controlled via eitherfour or eight data lines, a parameter whichneeds to be set immediately following switch -on. There is a choice of whether the data is tobe displayed with just one line of eightcharacters on screen, or two lines each ofeight characters. This factor, too, needs to beset after switch -on. So does the mode of

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screen shifting, for which there areseveral options. A further factor to beset is the screen position at which youwant the newest character to bedisplayed.

Whatever the number of screen ordata control lines chosen, you haveaccess to the entire character libraryheld in the lcd's permanent memory.Any of these characters can betransferred to the screen memory.Each screen line has an associatedtemporary memory which can store upto 40 characters. Under full modulecontrol, as described last month, anyconsecutive group of eight charactersfrom the total store can be displayed.

The module has three other controllines, RW, RS and E. RW sets whetherone can write to or read from themodule. Here we are only interested inwriting to it, so the RW line is heldpermanently low. The RS line tells the


Fig 6. Wiring of VR1-VR3.

CONSTRUCTIONThe pcb track and component layouts are

shown in Figs. 2 and 3. No special assemblytechniques are required; just ensure that youuse is sockets for all ICs 1-7, and that thepolarities of the electrolytic capacitors, diodesand the chips themselves, are observed. Thewiring for VR1-3 is shown in Fig.6, and thatfor the lcd module in Fig.7. The inverterreferred to in Fig 7. is an optional backlightdriver, described in the PE June LCDMessage Maker article.

The hex dump of the eprom data is shownin Fig.5. You will, of course, need to use aneprom programmer to program the data intothe eprom, but that should be no greatproblem in view of the variety ofprogrammers published in PE over the lastcouple of years. In my own unit I used aneeprom type 2816 (actually a ZicorX2816AP-35, though another manufacturer'sequivalent to this should be ok). If you wantto use a normal eprom, types 2716 or 27C16are suggested.

IN USESome possible signal sources and the level

range conditions were given earlier in the text.Once a suitable signal is being fed into thedecoder you must set VR1-3 to suit the signaltransmission rate and periods.

I found the best method was to switch Siso that the dots and dashes were displayed onthe screen, and then turn all three control pots

to maximum resistance (fully clockwise).VR1 is then slowly decreased until the dotsand dashes on the screen correspond withwhat you are hearing from the signal source.Now slowly reduce VR2 until other screencharacters begin to be interspersed with thedots and dashes. This will correspond with thesecond monostable, IC5a, being roughlytimed so that letter pauses are beingrecognised. Finally, once you recogniseconsistency in the decoding shown on screen,reduce VR3 until breaks between words areshown as blanks on the screen.

Note, of course, that you may not alwaysunderstand the decoded messages since theymay not be in English, or may have beenencrypted prior to transmission. Languagetranslation and de -encryption is beyond thescope of this project!

Anyone wishing to find out more about theRadio Amateur's examination and certificateshould write to The City and Guilds ofLondon Institute, 46 Britannia Street, London,WC1X 9RG.

RSGB MORSEThe Radio Society of Great Britain

transmits morse code at a slow rate on variousfrequencies in many parts of the UK. Fig.9shows a list of the areas covered by the slowmorse transmissions, together with thefrequencies and times at which they usuallyoccur. Readers who would like to find outmore about the RSGB should write to theRSGB HQ, Lambda House, CranbourneRoad, Potters Bar, Herts, EN6 3JW.

Fig 7. Pin connection for the LCM 570.

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HF TRANSMISSIONS

DAY TIME FREQ AREA

Mon 18.30 3.560 AvonTue 12.00 3.550Wed 18.30 3.550Thu 18.30 3.550Fri 18.30 3.550Sat 12.00 3.550Sun 12.00 3.550

Sun 10.15 1.875 GloucestershireMon 20.30 1.878 Greater LondonThu 18.30 1.078

Thu 20.00 1.414 Lincolnshire

Wed 21.30 28.350 StrathclydeThu 22.00 28.350Wed 19.00 28.450 West YorkshireFri 20.00 3.585

VHF TRANSMISSIONS

Tue 20.30 144.250 Bedfordshire

Mon 20.00 148.250 Co AntrimWed 20.00 146.250

Wed 18.30 145.250 Co DurhamFri 18.30 145.250

Wed 19.00 145.250 CornwallMon 19.00 145.250 DerbyshireThu 19.00 145.250

Mon 20.00 145.250 EssexTue 20.00 145.250Wed 20.00 145.250Thu 20.00 145.250Fri 20.00 145.250

Wed 19.30 144.250 Greater LondonSat 19.30 145.250

Tue 19.30 145.275 ManchesterThu 19.30 145.275Fri 19.30 145.275Sat 19.30 145.275Sun 19.30 145.275

Sun 20.30 144.250 Kent

Tue 19.00 145.250 LancashireWed 19.00 145.250Thu 18.00 145.250Fri 19.00 145.250Sun 12.00 145.275

Mon 21.00 145.250 MerseysideWed 21.00 145.250Thu 21.00 145.250Fri 21.00 145.250

Fri 20.30 145.250 Norfolk

Mon 20.00 145.275 Nottinghamshire

Wed 19.00 145.250 OxfordshireThu 20.00 145.250Sun 11.00 145.250

Mon 19.00 144.155 Somerset

Sat 10.00 145.250 StaffordshireSun 12.00 145.250

Mon 19.15 145.250 StrathclydeTue 19.15 145.250Wed 19.15 145.250Wed 21.30 145.250Thu 22.00 145.250

Mon 19.30 145.250 SurreyThu 19.30 145.250Fri 19.30 145.250Sun 19.30 145.250

Mon 20.00 145.250 Tyne & WearThu 20.00 145.250

Mon 19.30 144.160 West MidlandsTue 19.30 144.160Wed 22.00 145.250Sat 19.30 144.160Sun 19.30 144.160

Fig 9. Some RSGB slow morsetransmissions.


TTL S

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74537454746074707472747374747475747674807481

748347484474857486748974907491749247493474947495474967497747007410774109741107411174116741187411974120741217412274123741257412674128741327413674141741427414474.37414574147741487415074151474153741547415574156741577415974160741617416274163741647.657416674167741707417271173741747417574176701787417974180741817418274184741854741907.917419274193741907419574196741977419874199742217425174259742657427374276742787427974283742857429074293742987435174365474366474367474368474376743907439374490

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741_591

741092741593741.095974/59674151077415109741$112741011374011474151227415123741012474101257410126741.013274161337415136741.013874151397410145741_51477419148741015174101527410153741515474151567410156741615774161587415160474151614741.016247410163474101647415165474151664741516874161697416170741.517347410174741517574101817410183741519074101917410192741519374151444741.01954741019674101977419221741524074102417410242741.024374152447415245741024774152487415249741.02517416253741_0256741025747415258474102597410260741026174102667410273741027574102797410280741.528374152907415292741529374152957410297741029874102997410231741532374103247410348741535274153537415356741_53637415364741936574153667415367741536874153737410374741537574153777410378741537974163817415390741.5393741939547415399

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1.

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DAC1408 8 1000400800 1009400808 3.00

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6.750.95400COO/500901403.001001503.501.200.600.901.101.004.500.3(I0.050750.601501.800451.151101600403.60150oso3.00

1501.501002.003.251.002701601.100154.00cosOAS1.000.604.000.65022070

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117"°903

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VARIABLE REGULATORS14430548 2.501.3171 1.201613176 2.401143377 2.251443501 4.001M3960 15.00

OPTO ISO1107411074MC126MCS240044003020T11111

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0.506.502.256.752501.40

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MC14951.MC1496MC3340MC3401MC3403MF I OCN86502401.41.920

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M84662214NE53INE555NE556NE564NE565NE566555670NE570NE571NE592NE5532PNE5533PNE5534PNE55344POPO7EPP11024RC4 1 36R04151804795R045589502409441900SFF9636451490SN76033NSN764899576595SN7666097025681T.20T 47730TA7204147205747222TA7310TBA23 ITBA800T134810S784820154820M784920TBA95010910910494010410107041022TDA1024TO411700

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1001803.503403401502.304.502.001.50/750453909702009700.654.009005904.003001200.220.604.001201.501.254.003.000.901501.801201.501505.000.552901500.559.00

16.008.001003.00COOCOO1.207.001.201.401.500.901.501.501200.800.900.600.752.002.255001.752.251.501.10300

TD42003TDA2004TDA2006TD42020TD420301014381070470001E41002T1.061CP1106211064T10711107271074710817108211083T108471094U4759U42240U0145801401N20038U1_5200441.11142068

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002206XR2207082211X022160822402N4042541425419P25423EZN424E2N425EB25426EB2542703214428613

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1.902.403.203.202.507501507.000.400600900400.701.100.350.500.751.002.003.201.506.000.750.752.90lao

.801.902.752003005.004.004.003.75

5.755751.201.00

0.801.75

1.301.303.501001.004.502.259.007.503.007.503.002006.60

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CPUs1802CE 6.502650A 10.506502 4.5065002-2MHz 12.0065024 65065020 8.006800 2506802 1006E103 2 12 006809 6.506809E 1000

ATTENTIONALL PRICES ARE

SUBJECT TO CHANGEWITHOUT NOTICE

CPUsoo.68009E80359039800398080A8085A80085880868087-58087-880287880287.10808887418748TMS9980TMS9995Z80Z804,Z80070008

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SUPPORT DEVICES

26572816 3032423245652065226522465326551A68216802168296840688406850680506852685468854687581548155815682058212821682248226822882438250855148253C-582554C-5825682570.58259C-5827582790.582828283828482878288D875540 2 9TMS4500TM59901TMS9902TMS9911TMS99.Z8OPIO2804P IXZ8OCTC280407CZ8ODART2804DART280081AZ8OADMA2804510280871028080702BOBDART

12 002000

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6.0014.005.005.00

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COO

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1.751.501.001.401.501.002.502001501.751.401.401.501.751.501.751.752.501.503.001.501251.602502002001501.701.501.501.751501/52.50

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CARAC TERENERATORFIO 3.2513

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0448131 6.00078304 3.50053691 3.50058830

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T he IBM PC and its compatibles arethe standard business micro-computers, and the features that have

made them popular amongst business usersare, in the main, equally valid for home use.Probably their main selling point is theenormous software base. The number ofprograms available certainly runs intothousands. If you can not find an IBM PCapplications program to suit your needs, thenit is quite likely that such a program is notavailable for any microcomputer. For theserious home computer user the mainattraction of the PCs is their expandability.Most PCs are supplied equipped withsomething like two serial ports and oneparallel printer -type port. Further expansion ispossible by adding "cards" into the expansionslots on the main circuit board (or"motherboard" as it is generally termed). Awide range of these cards are available, andyou can add practically any desired facility toa PC with a minimum of fuss and effort.Furthermore, do-it-yourself cards are

perfectly possible, making the PCs an

attractive proposition for the electronicsenthusiast. (We showed one example of aninterface board in PE April 90. Ed). PCs areprobably more expandable and customisablethan any other popular microcomputer.

ill! it 1 1 I 11-1-1it-5. 'i= 1" Iirriiritillit-, i-c,--1'1"-\i)ti'l'iiiiiiii. j-% -f=1 1iiiiirI.iI.jitillIII fir II_

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The author's home-made XT, built from cheap parts and leftovers.

DIY PCSIt would not be fair to give the impression

that the PCs are the perfect home microswithout any drawbacks. For games use theyare probably not as good as the Amiga andSTs, and fall short of certain 8 bit computersin some respects. In particular, the soundgenerator is a basic single channel type whichfalls well short of the standards that can beachieved by the Amiga, or the humbleCommodore 64 come to that.

The graphics capabilities of the originalPC were non-existent, but these days there areseveral common graphics options to choosefrom, plus a number of specialised graphicsboards. The cheaper options do not comparetoo well with most other 16 bit machines,offering more the type of thing found on 8 bitcomputers (but with the advantage of 16 bitprocessing power). The more up-marketgraphics boards offer the sort of performancethat the Amiga and STs are hard pressed tomatch, but at present these take the machineinto a different price league.

Often the inadequacies of PCs areexaggerated by quoting items from thespecification of the original PC. Current PCsoffer something more potent that a 4.77MHz8088 microprocessor with 64K of ram and acassette recorder for storage. Most have thefull 640K that can be used by the MS-DOSoperating system (possibly with some form ofmemory expansion taking them beyond640K), an improved processor running at8MHz to 33MHz, and at least one floppydrive. With the "giveaway" prices of harddisks for PCs a substantial proportion of themare equipped with one of these devices.

Robert Penfoldshows that

computing yourown computer isnot inconceivable

with a PC.

PC MAKE-UP

All the major parts for a PC are readilyavailable, and building your own PC is aninteresting project to undertake. I must stressthat we are not talking here in terms ofmaking your own pcbs, soldering all thecomponents onto them, etc. A diy PC can insome cases be put together without the needfor any soldering, since the power supply,motherboard, etc, are all available ready -assembled. It is a matter of putting togetherthe right boards, and so on, and configuringeverything correctly. It is not quite as easy assome articles and books would suggest, andthere are a few pitfalls to avoid. In this articlethe main problems, and how to avoid them,will be explained. Apart from being of interestto potential diy PC builders, this informationshould be of value to someone who isthinking about buying a ready-made PC, andto those who may need to do occasional PCrepairs and upgrades. It also provides some

useful background knowledge for those whowish to undertake PC oriented constructionprojects.

A PC consists of about a dozen majorparts, and here we will consider each one inturn.

XT MOTHERBOARDS

There are two main types of PC, which arethe XT and AT compatible types. The XT iseffectively the original form of the computerwith an 8088 microprocessor, but modified totake more on -board ram and to permitoperation with a hard disc. While the originalPC operated at a clock speed of 4.77MHz, allthe modern boards seem to be of the "turbo"variety. These mostly had 8MHz clock at onetime, but 10MHz (plus a few 15MHz types)now seems to be the order of the day. Someboards use the NEC V20 chip. This has fallenout of favour due to legal wrangles with Intel,the originators of the 8086 series ofmicroprocessors. Press reports would suggestthat NEC have won this battle due to Intel notputting copyright notices on their processors,and the V20 might come back into fashion. Ituses improved internal architecture to performsome instructions using fewer clock cycles,giving a speed increase over the 8088 runningat the same frequency. Some speed testswould suggest that the V20 is more than twiceas fast, but in practice the increase is likely tobe more in the region of twenty percent or so.

Although some XT compatibles (notablythose from Amstrad and Olivetti) use the

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An XT board using a VLSI chip to reduce it to about two-thirds the size of astandard XT board. In "turbo" mode it operates at 15MHz!8086 or NEC V30 processors, you areunlikely to find anyone selling a motherboardwhich has either of these. The 8086 and V30have 16 bit buses, giving higher performancethan the 8088 or V20 with their 8 bit data bus.Some clever circuitry is used to permit thesecomputers to operate with the standard IBM 8bit expansion slots.

The memory limit of the 8088 is 1

megabyte, but as a lot of the memory map isoccupied by roms, video ram, etc, this leavesonly 640K of the map free for the main ram.Most motherboards will take the full 640K,usually in the form of eighteen 256K x 1

drams (the 41256) for the first 512K. Theother 128K used to be mainly in the form ofeighteen 64K x 1 drams (the 4164). Thesedays it is more common to have four 64K x 4drams (the 41464) and four 64K x 1 drams.Some boards now take 1 megabyte of ram inthe form of nine 1M x I drams (41000 or51000 chips). The extra 384K of rampresumably operates under some sort ofpaging technique, and is unusable byprograms, but usually the motherboard will besupplied with software to permit it to be usedas a ram disk. In other words, the ram canoperate as a sort of instant access pseudo diskdrive, and this can be used to good effect withsome programs. For an 8MHz board 150nsdrams are usually satisfactory, but 120nsdrams will probably be needed for a 10MHzboard.

You really need to buy the motherboardfirst, and then get the memory chips whenyou are sure which ones are needed. Fittingthe ram chips normally entails no more thanplugging them into the sockets already fittedon the board. A few boards are designed to

take drams which have a single line of pins(usually called "sips"). Unless the boardcomes complete with the ram (which is

unusual), it is probably best to avoid these asit may be difficult to obtain the dram chips,and they could be quite expensive if you canlocate a source of supply. The cost of memory

chips in general and drams in particular roseenormously a few years ago, but prices doseem to be moderating a little at last. It isworth considering secondhand drams - I

obtained 640K of dram for about £75-00 thisway, and have not experienced any problemswith it. It is worth pointing out that mostmajor PC software requires a minimum of512K, and an increasing number of programsrequire the full 640K.

Apart from memory, the only otheressential you will need to complete themotherboard is a rom bios (basic input/outputsystem). Most boards are supplied ready fittedwith a bios chip these days, but you mightfind that this is an optional extra. The bios isvery important since compatibility with PCsoftware is dependent on this doing a goodjob. Tried and tested makes are ALMI,Phoenix, and Award. Many XT boards seemto be fitted with bios chips produced by theboard manufacturer rather than a third -partysupplier. This could compromisecompatibility, but I have not found anyproblems using a board fitted with a rom biosof this type. It does ensure that changing theclock speed from the keyboard will work.Usually pressing "Ctrl", "Alt", and either "+"or "-" on the numeric keypad toggles between4.77MHz and the turbo speed. Speedchanging via a hardware switch is almostinvariably possible as well. Operation at theslower speed is retained because somesoftware will not run at higher speeds. Thisproblem is mainly caused by copy protectiontechniques.

The standard number of expansion slots iseight, but some boards have only five, six, orseven. This is acceptable if functions likesome of the ports are built onto the

This is a full-size "turbo" AT motherboard (most are XT size these days). It hasthe usual 6 x 16 bit plus 2 x 8 bit expansion slots.

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motherboard, as you will then not need toplug in so many cards in order to bring thecomputer up to a usable specification. Eightslots might seem to be more than will ever beneeded, but bear in mind that a few essentialfunctions (including the display driver anddisk controller) are not included on themotherboard. At least two slots must beoccupied in order to produce a usablecomputer, and three or four will be occupiedif you include a few serial and parallel ports.Also bear in mind that it can get a bit crowdedinside the computer, and life is much easierwhen adding your own boards if there areseveral spare slots available.

AT MOTHERBOARDS

At motherboards are somewhat larger thanthe XT type, although there seems to be atrend towards XT size AT boards which canfit into cases of either type. The original ATcomputer ran at 6MHz, and was latersuperseded by an 8MHz model. These daysAT compatible boards usually have one orboth of these speeds plus at least one "turbo"speed. This usually means a 10, 12, or12.5MHz clock rate. This may not seem to bemuch better than a fast XT, but remember thatthe 80286 of an AT board does more per clockcycle than the 8088 of an XT board. Also, its16 bit data bus aids increased speed. A12MHz AT board is therefore a few timesfaster than a 10MHz XT board. You need tobe careful when comparing AT boards sincesome have zero wait state operation, whileothers add in one wait state so that thememory chips can keep up. The zero waitstate boards are significantly faster, but needfaster and more expensive ram chips.

While AT motherboards are in many wayssimilar to XT boards, there are someimportant differences. Most AT boards haveeight expansion slots, consisting of two 8 bittypes and six 16 bit slots. Note, though, that a16 bit slot is just an 8 bit type with anadditional edge connector which carries theadditional data lines plus some other lines.Most 8 bit cards can be plugged into any ATexpansion slot without difficulty (but 16 bitAT cards can not usually be used with XTboards, even if they can be physically pluggedinto place).

XT boards have an octal dip switch whichtells the computer certain configurationinformation, such as the amount of ram fittedand whether or not the maths co -processorsocket is occupied. Most XT compatibleshave dip switch arrangements which areclosely based on the IBM original (Fig.1).The maths co -processor, incidentally, cangreatly speed up programs that are written totake advantage of it (mainly spreadsheets andcad programs). A few programs will not runwithout it, but most software does not makeuse of it. All XT and AT boards seem to havea socket for their respective 8087 and 80287maths co -processors. Returning toconfiguration information, on an AT boardthis is contained in the cmos ram of the 6818battery backed clock/calendar/ram chip. A

Octal DIP Switch

ON 1 2 3 4 5 6 7 8 ON

eeoeeooeOFF OFF

Fig 1. The function of the XTdip switches.

Number Of Disk Drives

Disply Type

Amount Of RAM Fitted

Maths Co-Processor

Normal Operation

setup program is needed in order to put thisinformation into the 6818. Without thisprogram an AT can not be put into properworking order, and amongst other things willnot recognise any hard discs that are fitted.Fortunately, many bios chips have a built-insetup program these days, and aPD/Shareware setup program is available.Being a true 16 bit machine, an ATcompatible has two bios roms (two 8 bit typesgiving 16 bit wide data overall).

The 80286 can operate in 8086 emulationmode (the normal MS-DOS mode), or in amode that gives a 16M address range. CurrentAT boards seem to take a minimum of 1M,and using 1M chips plus 1M plug-in memorymodules some can take as much as 8M ofmemory. MS-DOS can utilise this "extended"memory as ram disk, and some software canmake direct use of it. Do not confuse"extended" memory with "expanded"memory. The latter is a form of memoryexpansion for any PC, usually accomplished

using ordinary expansion cards. However, afew recent AT boards can have the extra ramconfigured as LIM 4.0 standard expandedmemory.

80386 based motherboards are widelyavailable, and are effectively AT compatibleboards based on the 32 bit 80386microprocessor. They have the advantage ofhigher clock rates (usually 16, 20, or25MHz), but when running ordinary MS-DOS software their 32 bit architecture is notproperly utilised. Most have a couple of 32bit expansion slots, but as there is no true 32bit bus standard, these might not be of muchuse. For many purposes one of the new 16 or20MHz AT motherboards will work just aswell. However, for the diy PC builder anyclock frequency of more than about12.5MHz is a bit risky, as not all cards willwork with these, and the fast ram needed todo full justice to these boards is not the typeof thing you are likely to be able to obtaincheaply.

Interior view with some of the cables removed, showing the general layout of a PC.


COMPUTING FEATURE

Plug-in cards can provide the PC with standard and unusual features.

CASES AND PSUs

Most XT cases are now of the flip -toptype, which is handy if you need to open thecase a lot to delve around inside. An AT caseis somewhat larger and most have an outercasing which pulls forwards and clear of thechassis, after removing five screws from therear panel. There are also "baby" AT cases,which are often of the flip -top type. Many ATmotherboards will fit an XT case, but there isa lot to be said in favour of a full size ATcase, It gives plenty of space for everything,and generally makes life easier. An AT case ispretty huge, but you can get inexpensivestands which enable them to be floor mountedon one end. You can then tuck the unit awayunder the desk, leaving the desktop free forthe keyboard etc. In fact you can get properfloor mounting "tower" cases, but these aremostly quite expensive at about four times thecost of an ordinary AT case.

When buying a case it is important to makesure it is supplied "with kit". In other words, itshould come complete with any odd bits ofhardware you might need, such as theloudspeaker, disk drive mountings,motherboard mountings, and so on. Whilesome of these odds and ends are standarditems, some of them are special plasticmouldings which it would be difficult for thediyer to produce. With an XT the motherboardis bolted to nine threaded mounting pillarsfitted on the base of the case (only six may beneeded if the board is a small type). ATmotherboards are mostly fitted in place using amixture of plastic stand-offs and threadedmounting pillars. An XT case has a cradle intowhich up to four half height drives can bemounted directly. The AT method is to haveplastic rails which are bolted to the sides of thedrives. They then slide into the mounting bays,and are held in place by clips bolted to thefronts of the mounting bays.

The standard power supply for an XT is a150 watt type. This is sufficient to power anXT board, floppy drives, a hard drive, and anumber of expansion cards. An AT has a 200watt power supply, although 180 watt typesare sometimes used with modern vlsi boardswhich have lower current consumptions.80386 computers mostly use a 220 or 230watt supply. These are all fully enclosedswitch mode supplies with a built in on/offswitch. They are fixed to the rear panel of thecase using four bolts. The case should beready drilled and cut to take the power supply,and everything else for that matter. The powerinput is via a standard Euro mains connector,and there is usually a switched Euro outlet forthe monitor.

FLOPPY DRIVES

The standard floppy drive for an ATmachine is the 1.2M 5.25 inch type, but theseare now being superseded by the 1.44M 3.5inch drives. Similarly, the 360K (40 trackds/dd) 5.25 inch XT drives are now beingdisplaced by the 720K (80 track ds/dd) 3.5inch type. The 5.25 inch types are the saferbets as not all software is available in 3.5 inchformat, although there is now very little that isnot. Special mounting kits are needed for 3.5inch drives, and are sometimes supplied withthe drive. You need to be a little careful whenselecting drive types, since everything willonly work properly if the bios and diskcontroller both support the drives you will beusing.

Apart from minor mechanical fittingdifficulties, probably the most commonfloppy drive problem is the setting of thejumper leads. Most disk drives (and manyexpansion cards) have terminals which can beconnected together, or not, depending onwhether little "jumper" clips are fitted ontothem. This enables you to configure thingscorrectly. Most disk drives seem to have about

a dozen sets of jumper terminals, but are notusually supplied with information on theirfunctions. Your supplier should be willing toprovide a data sheet or set up the drive readyfor use in a PC. There is a potential problemin that the genuine twin drive IBM PCs haveboth drives configured as drive 0. The lead tothe second drive has a twist which reverseslines 10 to 16, and effectively configures thesecond drive as drive 1. Not all clones use thissystem, and you might be supplied with drivesset up as drive 0 or I. Provided you get theright setting for the appropriate jumper, leadswith or without the twist should be usable.When fitting a single drive to my home-madeXT I must admit that I could only get it towork properly with the twist included in thecable. The standard floppy drive cable is a 34way type fitted with an IDC connector at thecontroller end, and a 34 way IDC edgeconnector at the drive end. Pin numberingshould be shown on both the controller cardand the drive, leaving no excuse for gettingone of the connectors fitted the wrong wayround. For twin drive operation another 34way edge connector must be added a fewinches from the drive end of the cable.

HARD DRIVES

Assuming you do not opt for an exotichard disk drive, this will be a type having anSCSI interface. This has a 34 way cable forcontrol purposes, and like a floppy drive cableit has an IDC connector at one end, and anedge connector at the other. A 20 way cable ofthe same basic type is needed to carry thedata. Note that the XT and AT hard diskcontrol cards are very different. The XT cardincludes a rom bios, whereas an AT controllerdoes not need this as the bios on themotherboard handles the hard disk drives.

XT controllers usually have dip switcheswhich should be set to suit the type of driveyou are using. The number of drivessupported might be quite small, butsometimes a built-in program can be run, andthis will enable the card to operate withpractically any SCSI drive. Most XTcontrollers operate by storing the disk's vitalstatistics on the disk itself. This may seemlike a genuine analogy to pulling yourself upby your bootlaces, but it works! Thecontroller should include a formattingprogram which places this information on thedisk, as well as performing a low level format.A low level format is what could be regardedas the real format, where the track and sectorinformation is written onto the disk. At thisstage any defective tracks must be "mapped"so that they are never used. A hard diskshould be supplied with a test sheet givingdetails of any defective tracks, performanceinformation, number of tracks ("cylinders")and heads, etc. This sheet should provide allthe data you will need to successfully performa low level format.

At least, it will tell you everything but thecorrect interleave factor. I will not go intogreat detail about interleaving here, but it is

basically a matter of arranging the data on the


The original 84 -key keyboard - now probably less commonly used than the 102 -key keyboard.

disk so that when one chunk of data has beenread and transferred, the next chunk is almostinstantly available, avoiding the need for thedisk to do almost a complete rotation beforethe head is positioned at the beginning of thenext piece of data. This can significantlyspeed up transfers of large amounts of data.For an AT the interleave factor is 2, exceptwhen using a "turbo" AT with a specialcontroller having a factor of 1. For an XT theinterleave factor is usually much larger atabout 7 (and the formatter might simply set avalue without consulting you first).

The next step is to use the MS-DOSFDISK command to prepare the boot sector ofthe disk so that the FORMAT command canoperate properly. It can also be used topartition the disk, which means having iteffectively operate as two smaller disks. Thisis usually only done if the disk has a capacityof more than 32M, since this is the largest sizethat MS-DOS 3.3 (and earlier versions) canaccommodate as a single disk. Note that MS-DOS 4 apparently does not have thislimitation. Some hard disks are suppliedtogether with partitioning software, and thismight be easier to use in these cases. The finalstage is to use the MS-DOS FORMATcommand to do a high level format of thedisk. Presumably you will wish to boot fromthe hard disk at switch on, and so the /s option

should be used in order to install the MS-DOSsystem on the disk.

Installing a hard disk in an AT computer isa little different, since the controller does nothave any built-in software. For the diy pcbuilder there is a lot to be said in favour of arom bios such as a recent "AMI" one, whichincludes a format program, setup program,diagnostics, and details of the supported diskdrive types. Any AT bios should support atleast drive types 1 to 15, and some go as highas type 47. However, there seems to be a lackof true standardisation from types 16 to 47.Probably the most common type of drive istype 2, which is the one most 20M drives arecovered by. If you opt for something moreexotic than a 20M drive you could well findthat nothing in the list of drives exactlysupports your unit. This is not necessarily adisaster, and it is quite common to choose anear equivalent, although this might meansacrificing some of the drive's capacity. Youmust select one having the same number orfewer heads, and the same number or fewertracks. In a rebuilt AT I used an 853cylinder/6 head drive as an 820 cylinder/6head 42M type (drive type 40). This losesabout 2M of the drive's capacity, which is apity but of no great importance.

Preparing a hard disk for operation in anAT is much the same as for an XT. However,

This 102 -key layout is probably the most popular type of PC keyboard at present.

, CL -4_7.:#111-.\ \ \

,..,I.:- ir k\

'` '-

,- : , - , \ s

k ' \ \ \1

k \' \)- \ \ \

-. "( \ \,..:-.. ,

--4--- - .

initially the setup program must be used toindicate that no hard disk is fitted, or thecomputer will probably refuse to go throughits start-up routine properly. Use the setupprogram to indicate that the hard disk ispresent once the low formatting has beencompleted.

Most hard disk controllers are of the mfm(modified frequency modulation) type. Theseoperate on the basis of sectors per track with512 bytes per sector. RLL (run length limited)controllers are now becoming more common,and these use 25 or 26 sectors per track,giving about 50% more capacity and fasterdata transfer rates. Note though, that r11

controllers should only be used with drivesthat are designed for this higher densitysystem. Using an r11 controller with anordinary drive, as many users can testify, islikely to give very poor reliability.

KEYBOARDS

The 84 key IBM style keyboard is thesafest option, but it is less popular than the102 key type which has 12 rather than 10function keys, and separate cursor keys etc.Instead of having these merged into thenumeric keypad (with a key to toggle betweennormal and numeric operation). Relativelylittle software utilises the extra function keys,and I have not encountered any programswhere they are essential. There is also a 101key layout, but this does not seem to be verypopular in the UK. There is a potentialproblem with the 102 key type in that a fewkeys usually produce the wrong on -screencharacters (the # and \ keys often providingtroublesome). Ready assembled computersare usually supplied with a utility program tocorrect this, but commercial, pd, or sharewarekeyboard definition programs can usually sortthings out. Although XT and AT keyboardsuse slightly different methods of interfacing tothe main unit, all the IBM compatiblekeyboards I have seen have been switchableto operate with either type of computer.

DISPLAY ADAPTORS

The lowest cost display adaptor/monitorcombination is the Hercules monochrometype. This gives good quality text plus highresolution (720 x 348) monochrome graphics.This is adequate for many purposes, but thereis a great weakness for home use in that littlegames software supports this mode. For homecomputer use the cga (colour graphicsadaptor) is probably the best budget display.This provides 640 x 200 monochrome and320 x 200 x 4 colour graphics, plusreasonable text capabilities. Virtually allgames software, plus most serious softwaresupports this mode.

By current standards neither of thesedisplay standards are particularly outstanding.The ega (enhanced graphics adaptor) type is adefinite improvement, with its 640 x 350resolution in monochrome or 16 colours.Although at one time ega cards and monitors


COMPUTING FEATURE

were extremely expensive, they are now anattractive proposition at prices not that muchmore than those for a cga display. EGAsystems provide the cga modes as wellincidentally. VAG (versatile graphics array)displays are becoming very popular, but arestill relatively expensive. In addition to all thecga and ega modes they also provide a 320 x200 mode offering up to 256 colours from apalette of 262,144 colours, and a 640 x 480 16colour mode. In fact many offer even moreimpressive modes, such as 800 x 600 in 16 or256 colours. In order to make use of thesemodes you need an expensive multi -syncmonitor rather than a vga analogue type, plussoftware that supports them, or (moreprobably) suitable drivers for yourapplications programs supplied with the vgaboard.

MS-DOS

In order to use a PC it must boot up anoperating system at switch -on, and by far themost popular operating system is MS-DOS. Intheory, MS-DOS is only sold with readyassembled computers, and is eithercustomised to provide complete compatibilitywith the hardware, or is thoroughly tested toensure that the standard version operatesproperly with the hardware. In practice, a fewcompanies sell MS-DOS, usually version 3.3,but a few offer version 4. Either will do, as

will any version from 3.0 onwards (not allprograms will run under version 2).

PORTS

With few exceptions, motherboards do notinclude any ports, and these must be providedby expansion cards. Probably the cheapestway of providing these is to buy a multi-function card. In my home assembled XTcomputer I use a card which cost £39.00(excluding vat). It provides parallel, serial,and games ports, plus the floppy diskcontroller and a clock/calendar. A chip boughtfor about £12.00, together with two chips plussome cable and connectors from the sparesbox, added a further serial port. Together withthe parallel port included on the cga card, thisgives the machine an impressive array ofports at minimal cost.

GETTING ITTOGETHER

Putting the computer together is notparticularly difficult, and is reminiscent ofassemble -it -yourself furniture. You may needto supply one or two bolts, and it might benecessary to make up a few cables. However,ready-made cables are available, the powersupply has flying leads fitted with connectors,and hard disk controllers often come complete

with a pair of cables. The accompanyingphotographs help to show where everythingfits, but it is fairly obvious anyway once youhave all the parts. The main point to watch isthat you have all the information you need toset up dip switches and jumpers correctly.Ideally, every board and drive should besupplied complete with at least a pamphletgiving any necessary setting up information.In some cases software might be needed aswell, such as to enable a clock/calendar to besetup and used correctly (my XTclock/calendar has yet to do anythingworthwhile due to a lack of software to set itup and integrate it with the operating system).Before buying anything it is a good idea toask lots of questions to ensure that the itemdoes what you think it will, and is suppliedwith any software/information that you willneed. You are almost certain to find a fewproblems to solve, but that is all part of thefun!

Assembling your own PC is certainly aninteresting pastime. My XT was built fromcheap parts plus a lot of left-overs fromupgrades to my other PC. Its cost was about30% of that of a ready assembled equivalent,although without the left-overs the savingwould obviously have been much less thanthis. Having completed your PC, you have theideal basis for computer projects. Using PCswith some constructional projects will be thesubjects of future articles in PE.

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26 PRACTICAL ELECIRONICS JULY 1990

AUDIO PROJECT,

For some while now the electretmicrophone has enjoyed increasingpopularity as an inexpensive

instrument capable of good quality.Basically, it is similar to the condenser orcapacitor microphones that have been usedfor decades in broadcast and recordingstudios. These consist of a thin plasticdiaphragm coated with aluminium or goldstretched over a shallow cavity having a flatmetal backplate. A capacitance is therebycreated which depends on the spacingbetween the diaphragm and the plate.

An applied dc voltage usually of 48Vcharges the capacitor and electrostaticattraction pulls the diaphragm in, keeping ittaut. Movement of the diaphragm caused bysound waves varies the capacitance andcauses current to flow in and out of thedevice in sympathy. The current is passedthrough a load resistor thereby developing asignal voltage across it, but because thecurrent variations are very small, a largevalue resistor must be used to get anappreciable signal output.

The output impedance is thus very high,in the region of 100 M , and therefore a cablelength of more than a few inches would

Fig 1 (a) phantom powering using resistors across input transformer;(b) phantom powering with centre -tapped transformer.

This is an improvement over the moving -coilunit which resonates at around 2 kHz and forwhich little can be done. The electret istherefore very suitable for public addresssystems where response peaks can initiatefeedback, as well as for other applications.

exercising a firm pressure. Because of thehigh amplification following it, any smallcontact resistance variations generate largenoise signals.

Another source of noise is the on/offswitch which is to be found on most electret

REMOTE POWEREDELECTRET MICROPHONES

cause severe loss of high frequencies due tocable capacitance. A built-in preamp to serveas an impedence converter is essential.

A power supply to provide the 48Vpolarising voltage and also to power thepreamp is required, and this is supplied fromthe mixing desk via the microphone cables.

ELECTRET

Enter the electret microphone. This worksin just the same way as its up-marketprofessional relative, but a permanent chargeequivalent to about 100V is implanted in thediaphragm during manufacture by heating theplastic between the plates of an air -spacedcapacitor. The charge leaks away in time, butit is claimed that with normal use it takes 100years to fall to half its original value. As noneof them has yet been around that long, thisclaim cannot be substantiated. It is knownthough that excessive moisture due tocondensation can greatly reduce the life.

The diaphragm of the true capacitormicrophone is so light that not only does itrespond well to high frequencies, but itsresonance peak is usually above the normalaudio range, thereby imparting a completelyflat frequency response. The electretdiaphragm needs to be thicker than this inorder to retain the charge, so the larger massresults in a lower resonant frequency.

Its resonant peak is usually between 5-8kHz which can be tamed with a little top cut.

Vivian Capelreveals another"phantom" of

the opera!

POWERING

The 48V required to polarise the studiocapacitor microphone would be a drawbackfor humbler applications, but with theelectret it is not needed because of the in-built charge. However, the preamp is stillrequired and must be powered.

To eliminate the need for externalpowering, electret microphones contain apreamp designed to run off 1.5V, andtherefore a single cell battery can becontained within the microphone. Themicrophone can be used in place of a movingcoil on any conventional mic input socket.

This convenience, though, has itspenalties. The current drawn is small sobatteries last a long time, consequently, sincethe contacts are undisturbed for lengthyperiods, they tend to oxidise. While frequentcleaning helps (providing both contacts areaccessible) it does not solve all contactproblems. They can lose their springinessand make merely a 'touch' contact instead

microphones. Contacts oxidise and aredifficult to reach for cleaning. An additionalhazard for pa operators is that users tend toplay with the switch and inadvertently switchthemselves off without realising it.

A further possibility is that a battery is leftin too long and fails just on an importantoccasion, or a faulty one can leak andcorrode the inside of the microphone beforethe damage is discovered.

As a result of these problems, manyoperators have abandoned the electret inspite of its superior performance, and goneback to using dynamic microphones. It wasthis dilemma that encouraged thedevelopment of a means of powering electretmicrophones from an external supply via thecable, like the studio models, and so dispensewith the troublesome battery.

To see just how this can be done we willfirst take a look at how the studiomicrophones are powered.

PHANTOM POWERING

Firstly, if we consider the unbalancedcircuit commonly used for domesticrecorders, it is found that the cable has oneconductor and a screen. The screen is earthedat the input socket and forms the return pathfrom the microphone. This disadvantage withthis is that, being in series with themicrophone circuit, any hum or noisevoltages induced in the screen are applied to


the input in series with the microphonesignal.

To avoid this, all professional and serioussound systems use a balanced circuit that hastwo conductors inside the screen. Thesecarry the microphone signal to a transformerwinding which is isolated from earth. Thescreen is earthed at the mixer end and also tothe microphone casing, but forms no part ofthe microphone circuit. The conductors arealso twisted together to cancel any hum fieldthat may penetrate the screen.

In conveying the polarising voltage to themicrophone, the trick is to do so without anyextra wiring, to make use of the existingsignal wiring. One method is called phantompowering. There are two versions, as shownin Fig.!. The first has a couple of resistorsacross the transformer winding with thejunction connected to the positive supply.The negative goes to the screen.

At the microphone end there is a centre -tapped transformer which has its centre -tapgoing to the positive of the preamp. Whilethe screen is taken to the negative. Thepositive supply current thus flows from theresistor junction, along both conductors andout through the transformer centre -tap. Thenegative return flows along the screen. Thealternative version uses a centre -tappedtransformer at the mixer in place of the twoshunted resistors, but the principle is exactlythe same.

Supply voltage is thus conveyed along thesignal cables without interfering with thesignal itself, but there is a snag. The screen isin series with the microphone supply, so anyrandom voltages it picks up can modulate thesignal. Really, we are almost back to thedisadvantages of the unbalanced system.

A B POWERING

The disadvantages of phantom poweringare overcome by A B powering as shown inFig.2. Here, the positive and negative of thesupply is conveyed by the two signal wires,and the screen is completely isolated as itshould be.

To prevent the supply being shortcircuited by the transformer, the winding isactually a split winding having a capacitor atits junction. The feed is via two resistors. Atthe microphone, a capacitor -coupled splitwinding is also employed, and the supply to

91

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Fig 3. Balanced remote AJB powered input circuit for one channel. A three -contact jack plug is shown, but a Cannon or any other three -contact type couldbe used.the preamp is taken from the inner ends ofthe windings.

POWERING THE ELECTRET

Having some idea of how theprofessionals do it with studio instrumentscould suggest a practical method of remotepowering for electrets. As most electrets areunbalanced, the conversion can bestow theadditional advantage of balanced operationas well as remotely powering themicrophone.

The model used for the prototype was theAltai EM 506, an excellent unit with ameasured hypercardioid polar response(although the makers claim only cardioidcharacteristics) and an even frequencyresponse. Other models could be converted ifthey were physically suitable. The EM 506has a four pin socket at the base of thehandle. This, or a three -pin, is required toconnect the twin -plus -screen cable. Mostelectret capsules are similar to the two typesused in this model.

The simplest method would be to use twinscreened cable with an unbalanced circuit.One conductor would be used for the signaland the other for the positive supply. Thescreen would be the microphone returncircuit as well as the supply negative. Forapplications using short leads where noiseand hum pickup are less likely, this may

Fig 2. AB powering. This avoids using the screen in the power circuit soreducing possible noise and radio pickup problems.

prove satisfactory, but a little extra troubleand a few more components can give all theadvantages of a balanced system.

At first, the use of transformers wasconsidered, but the microphone barrel wasjust too small by about 1 mm toaccommodate the smallest transformeravailable, and there was not a large selection.Availability of split -winding transformers forA B powering was even less. Cost too washigh, seeing that a transformer was neededboth for the microphone and for the inputcircuit, and that six microphones wereneeded for the particular pa system involved.

An electronic balanced input circuit wasdesigned some years ago and has been insuccessful use ever since, so it was decidedto modify this to give balanced A Bpowering in preference to the phantomsystem. The input circuit is given in Fig.3.Signal input from the microphone is appliedacross the base/emitter junction of TR1 viaCl and C2. C3 is included for rf suppression.The supply positive connection is fed to onesignal conductor from a 12V source throughRI, and the negative through R2.

The transistor is the low -noise BC109Cwhich is rc coupled to the second image.Output goes to a fader and from there toconventional mixer amplifier and outputstages.

MICROPHONE MODULE

The circuit for the microphone module isshown in Fig.4. The signal from the fetsource and drain is taken through Cl and C2to the A and B signal pins. These also carrythe supply voltage which is fed to the fet viaR3 and R4. The voltage drop over these andover RI and R2 of the mixer input provides avoltage of just over 1.5 volts from the 12Vsupply.

Note and observe the polarity of C 1. Thenegative goes to the drain in spite of itspositive polarity, because the other side atpin A is of a higher positive potential. R2and C2 are already mounted in the


AUDIO PROJECT LPJ

Fig 4 (left) (a) Circuit of microphone module for type 1 capsules having aseparate electret and fet. The mic socket is viewed from the free end of the pinsalso the solder tag ends of the plug. (b) Circuit for type 2 capsule havingintegrated electret and fet.Fig 5 (right) (a) Circuit of original type 1 capsule circuit before conversion.(b) Circuit of type 2 capsule before conversion. Notice that the capacitor wasconnected with incorrect polarity.microphone capsule in the case of type I

capsules as these formed part of the originalcircuit (Fig.5a).

With type 2 capsules the fet is integratedwith the electret unit, and the load resistor isin the positive leg (Fig.5b). They can bedistinguished by the appearance and layoutof the terminals and also by the differentresistor value, 560Q replaces the 82052.Whichever it is, the other on the panel shouldmatch it. For type 2 capsules, C2 and R2must be mounted freely from the capsuleterminal (Fig.7).

The first step after dismantling is toremove the small transformer at the base ofthe microphone which is included to give ahigh -impedance option. As no serious userever employs the high impedance mode it istotally useless. Next, the battery contacts canbe removed and also the wiring. Wiring tothe switch can be removed, but the switchitself should be left in place otherwise therewill be an unsightly hole in the body.

Having gutted the microphone, all isready for installing the printed panel. Theexisting wiring from the capsule may be

Fig 6. Microphonemodule Veroboardlayout. This just fitsinto the microphonebarrel in place of thebatterycompartment.

The diode DI is a protection in case thesupply is inadvertently reversed by wrongconnections to the plug, C3 decouples thesignal from the supply and also bypasses anyimpulsive noise on the supply voltage.

CONVERTING THEMICROPHONE

The removable barrel facilitates theconversion of the EM 506. The head alsoneeds to be unscrewed; this is the hardestpart as it is screwed on very tightly. A wellpadded vice and strap wrench may berequired.

Fig 7 (a) Connections from type 1capsule to module. (b) Connectionsfrom type 2 capsule. C2 and R3 mustbe suspended from the terminal asshown. These components wereinside the type 1 capsule.

usable. If not or if some extra may be neededthis should be fitted next.

The socket has four connections whichwere intended to give the choice ofunbalanced, high or low impedenceoperation, by reversing the connecting plug.This should be connected as shown with theearth lead going to a diagonal pair, as it wasoriginally. The A and B go to the otherdiagonals. The female plug has two keywaysand so can be connected two ways. To ensurecorrect connection, either replace the plugwith a standard one having a single keyway,or block one keyway on the original. Thiscould be done by filling it with Aralditeepoxy resin after first scratching its smoothwalls to provide a key.

Now ensure that the cable connections arecorrect. With the keyway at the bottom, theA positive connection goes to the top leftviewed from the plug solder tags, and the Bnegative to the bottom right. The sameconnections are made to the microphonesocket viewed from the free end of the pins,so when soldering inside, the A connectiongoes to the top right and the B to the bottomleft. Either three -contact jack plugs orCannon connectors can be made to the mixer.If using jacks, the A connection goes to thetip, the B to the ring, and the screen to thebody.

The result will provide microphoneshaving excellent acoustic properties, butwithout all the problems arising from internalbatteries.

PF

Microphone -feedback redefined.


PE RADIO CLOCKTuner unit Kit 298.1 £13.00Decoder Unit Kit 298.2 £36.50

More details In catalogue

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Many more inour catalist!Send medium sizedstamped addressed

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30 PRACTICAL ELEC1RONICS JULY 1990

HISTORY OF TECHNOLOGYr

[it is many years now since the name Hertzwas adopted as the unit of frequency. It isfitting that this was done for a brilliant

mah who gave so much to furthering sciencebut died so young. It was he who laid many ofthe foundations of our modem radio technologyand enabled it to be what it is today.

YOUTHFUL YEARS

Heinrich Rudolf Hertz was born on the 22ndFebruary 1859 in Hamburg. His father was asuccessful lawyer and he also had three brothersand a sister.

The young Hertz started school at the age ofsix and made steady but not outstandingprogress. However, later in his school life anaptitude for practical subjects became apparentas did his liking for languages. At the age of 18he moved to Frankfurt where he studied for thestate examinations. A major part of thesestudies included science and mathematicswhich he soon found he liked.

After Frankfurt, Hertz spent a year in Berlinfor his military service. Then he moved on theMunich and entered University to studyscience. This he greatly enjoyed, but in order tofurther his studies he transferred to Berlin. Herehe met the famous Herman von Helmholtz whowas to have a great influence on him.

Ian Poole's thumb-nail history revealsa giant step - anda great loss - for

mankind.

FAME

With the publication of the results of hisexperiments and the many demonstrations hemade, Hertz soon became famous. He wasoffered the position of Professor of Physics atthe University of Bonn which he took up in1889. Here is continued his research, but thistime he started to investigate the discharge ofelectricity in rarified gasses. He continued topublish papers on his work and reinforced hisreputation as one of the foremost researchersof his time.

In addition to this he received a number ofhonours from the various scientific bodies.One of these was from the Royal Society inLondon.

TRAGEDY

However, while Hertz was still at his primehe started to suffer from ill health. Hefrequently had headaches and was oftendepressed. Inspite of the fact that his doctorscould not diagnose the problem, he stillcontinued to work.

Slowly, Hertz's health began to declinefurther and at the end of 1893 he completed

HEINRICH HERTZAT UNIVERSITY

After Hertz arrived in Berlin it did not takehim long to get himself noticed. He did this bywinning a competition which had been set byHelmholtz; he showed that electricity had noinertia.

During his time in Berlin, Hertz built up agood relationship with Helmholtz whorecognised him as a very good student. Theresult of this was that after he graduated with aPh.D. magna cum laude, Hertz became anassistant to Helmholtz. During this time Hertzlaid the foundations for his career very well. Hepublished a number of papers on a variety ofsubjects and became well known and respectedamongst the scientific community.

RESEARCH STARTS

Despite the fact that Hertz was enjoying histime in Berlin, he felt he wanted moreindependence and this would require him tomove. This he did when he moved to KielUniversity. Unfortunately, there were very poorfacilities here and so Hertz had to contenthimself with a theoretical approach to hisresearch. He looked into Maxwell's famousequations, and saw how he could extend thescope of their application. In doing so he'prepared much of the way for his futurediscoveries.

Again Hertz felt he had to move on. In 1885

he moved to Karlsruhe Polytechnic to take upthe post of professor of physics. However, Hertzsoon found that there were other attractions toKarlsruhe as he met his future wife. The couplewere very happy together and they were soonmarried. Later, Hertz was to have two daughtersfrom the marriage.

At Karlsruhe, Hertz soon set about hisresearch. He based his work on his previousstudies of Maxwell's equations. However, hewas now able to perform practical experimentsto prove his work. This was particularlyimportant because many people had concurredwith Maxwell's equations and had agreed aboutthe presence of electromagnetic waves butnobody had been able to prove they existedexperimentally.

Hertz performed many experiments, but theone which is most often described today is theone in which he placed two loops of wire withina few metres of one another. Each loop had asmall spark gap and he showed that a sparkacross the gap in the first caused a spark tojump across the gap in the second. He alsoshowed that for the experiment to work the twoloops had to have the same dimensions.

Hertz did not stop here. He went on toinvestigate the properties of these waves. Hededuced their velocity and found that it wasalmost exactly the same as that of light waves.He performed other experiments and showedthat they could be reflected and refracted in thesame way as light. From these results heconcluded that beyond any reasonable doubtthey were the electromagnetic waves thatMaxwell had discovered mathematically.

his last book. Then on 1st January 1894 at theage of only 36 he died. This was withoutdoubt one of the greatest losses the scientificworld had ever known.

ADVANCING SCIENCE

Hertz had given much to forward thescientific knowledge of the day. In fact, radiowaves were called Hertzian waves for manyyears afterwards, but as they came into morecommon use the term slowly slipped away.Fortunately, his name has not been lostbecause in the late 1960s his name was givento the unit of frequency, a fitting but latehonour to one who gave so much to thediscovery and establishment of radio.

The fruits of the research which Hertz hadperformed were soon to be felt by the worldas a whole. People like Marconi were quick tosee the value of Hertzian or radio waves. theyrefined the experiments which Hertz hadperformed and made systems which could begiven practical uses. In fact it was Only tenyears after the death of Hertz that Marconi setup the first link between England andAmerica for the swift transmission of news.After this the whole idea of radio snowballedand it became part of modern life. One thingis certain: if it had not been for the insight ofHertz our radio technology would not bewhere it is today.

Photograph courtesy of The Institution ofElectrical Engineers.


T

0141115

h ink of a compact disc or cd whichcontains video, audio, graphics andtext and you have a cd-i disc. CD -I

stands for Compact Disc Interactive. Inventedby Philips and developed jointly by Sony andMatsushita, cd-i promises to become anextremely important multimedia platform, notonly for IT distribution and training but alsohome entertainment. cd-i effectively bringstogether the benefits of audio, video, computerapplications. As cd-i manufacturing uses thesame processes already in use for cd-audioduplications, costs will be low enough for themass market.

A cd-i disc has the capacity for 72 minutesof cd-quality sound - the same as a standardmusic cd, or 72 minutes of extremelycompressed digital video and sound, or 650megabytes of computer data - either in theform of raw text, programs or computerimages.

The video capacity of a cd-i disc isespedially impressive compared with justseven minutes video capacity on existing cdvideo discs. Data -compression and digitalinbetweening techniques having expanded D -Ivideo capacity without overly compromisingvideo or sound quality, which is said to becomparable to top -end vhs tape systemscurrently available.

CD -I will be launched to the public nextyear, and currently over 200 third partycompanies are developing systems andproducts around the new standard. Roughestimates point to a price of around £600 for acd-i player, which will be compatible with cd-rom and music cds.

At the recent Which Computer? showPhilips formally unveiled its first prototypecd-i machine (see top picture) which lookedmore akin to a desktop computer with itsmouse controller and tv monitor.

CD -I is expected to be attractive to traininginstitutions because, as its name implies, theuser will have considerable control over his orher cd-i system, being able to respond toonscreen prompts, go back and replay portionsof video and generally react interactively.

In the home, the uses of cd-i are virtuallyunlimited, from home education courses,electronic encyclopaedia, interactive gamesand of course pop videos and otherprogrammes.

HOME BASEInteractive communications areIan Burley's theme this month,

ranging from Philips' CD -I launchto yet more ambitious plans toget you rabbiting on the blower.

TRON INTELLIGENCE

Tron, the brainchild of Professor KenSakamura, from the University of Tokyo, isnearing birth. Tron stands for the Real-timeOperating system Nucleus and envisages afuture world where a large proportion ofeveryday electrical devices, both public anddomestic, will be linked and controlledthrough an intelligent computer network.

Fujitsu has just announced the completionof the third and final tron 32 -bitmicroprocessor, the Gmicro 300, which joinsthe Gmicro 100 and 200 models already inproduction by Mitsubishi and Hitachi. Workcan now progress on the development ofworking tron applications.

Everything from traffic lights to domesticheating could be controlled remotely andintelligently through a tron network.Sakamura's ultimate vision is for a whole cityto be built around a massive tron network.

The tron project was started in 1984 andquickly gained firm support from majorJapanese industrial giants, despite its fancifuland far reaching goals. Just six years later andtron's original sceptics are already lookingworried. Not just the home of the future butthe basis of the way communities of the futurecould be organised and run could be theresponsibility of the tron ideal.

BAE MICROTEL

The British Aerospace led Microtelconsortium has announced its plans to enterthe pcn (Personal Communications Network)race. Microtel unites British, French {Matra }and American {Pacific Telesis } hightechnology skills in a bid to provide low costmobile communications to the masses by theend of the decade. What has this to do with aconsumer electronics column? Quite simply,in a few short years pcns could threaten thefuture of today's old-fashioned wiredtelephone networks and unlike cellular phonenetworks, Microtel is predicting that pcns willbe affordable enough for nearly anybody touse. Indeed Microtel was talking bullishly of15 million pcn users in the UK alone by the

year 2000, with an ultimate target of half thepopulation into the 21st century. At today'sprices, Microtel predicts that compact pcnphones will eventually cost less than £100 tobuy and call costs will be substantiallycheaper than today's cellular tariffs. On top ofthat you will eventually be able to take yourpcn phone with you on trips to the continentwith the advent of a pan-European pcnsystem.

PCNs are not to be confused with the nowemerging telepoint cordless phone networks.Telepoint offers an enhanced alternative totoday's public call boxes, enabling users tomake outgoing calls at or near busy publicplaces. PCNs are more clearly going tothreaten existing cellular mobile phoneservices as they will offer incoming callcapability currently missing from telepointservices. However, both pcns and telepointhave one thing in common, they both transmitconversations using secure, interference anddistortion free, digital signals.


CONSUMER FEATURE*

Being digital, both pcn and telepointphones will benefit from advanced vlsiminiaturisation and lower power consumptionthan their conventional analogue counterparts.As telepoint has already shown, this enablesphones to be incredibly small compared withmost cellular phones. Microtel says its pcnhandsets will incorporate smart cardtechnology for storing user -id and billinginformation. Overlapping pcn base stationswill be strategically positioned all around thecountry with one metre radio antennas able tobe positioned inside buildings or under rooflevel in towns and cities.

Microtel is one of three competing serviceproviders licensed by the Department ofTrade and Industry and is committed toinvesting around £1.2 billion in setting up itspcn network. The others are Unitel andMercury. BT and Racal weren't eligible forpcn licenses as they already operatecompeting cellular phone services. InitialMicrotel pcn trials are expected to take placewithin two years, with the introduction of apublic service soon after.

Despite all the bullish optimism, there areplenty of problems to be overcome beforepcns are guaranteed success. Microtel, like itsother UK counterparts is going for the GSMnetwork standard though this has not yet beenunanimously adopted throughout Europe, andthe standard itself could require revisionsbefore pcns get going. Cellular networksshouldn't be written off. These are now wellestablished and new micro -cell techniques arealready promising to increase capacity andimprove the currently near -saturated service.As competition increases, so costs to thecustomer will come down. Telepoint has gotoff to a slow start, but this is likely to threatenpcns at the lower end of the market with evenlower costs than pcns. Finally, will Microtel'sambitious plans keep to schedule? The recordfor the introduction of telepoint and cellularservices isn't enviable. Only time will tell if

Microtel and the other pcn providers get itright this time.

BYPS TELEPOINTING

If there's one major criticism of the threeactive telepoint digital cordless phoneproviders, Ferranti Zonephone, BTPhonepoint and Mercury CaIlpoint, it's theremarkable failure of their marketing effortsto capture the attention of the ordinary man orwoman in the street.

The last DTI licensed telepoint operator tointroduce its service, the Barclays PhilipsShell consortium or BYPS, appears to haverecognised this point and come up with alively and imaginative marketing campaignbased around, of all things, a rabbit. The ideais that you use phones for rabbiting on - getit? OK, so it's corny, but at the recent Comms`90 exhibition it caught the imagination ofvisitors as no other telepoint campaign hadbefore.

At the moment, none of the telepointoperators, including BYPS is targeting private

home users directly. The phones and basestations at £200 a piece on average are still fartoo expensive. HoWeyer, BYPS is the first tosignal clearly to ordinary, non -business, usersthat telepoint is for them too, though maybenot for a year or two. By then the networkswill have expanded considerably and handsetswill hopefully have fallen below the magic£100 mark. Today's target is the disaffectedcellular phone user who might be fed up withthe Service or who primarily requires outgoingcalls only. BYPS also feels there is animportant potential market of many thousandsof people who carry electronic pagers.

Future promised enhancements to telepointinclude localised user -registration at sites, sayat work perhaps, where incoming calls can bediverted. This partially answers the criticismthat telepoint is non -viable without anincoming call capability away from yourpersonal base station at home. As for thethreat from pcns, BYPS says that these newlyannounced services won't be available to thepublic for up to five years, and in any casetelepoint will always command a considerableprice advantage over either pcn's or cellularservices.

Finally, BYPS, with its CPT designed andmanufactured handsets and base stations,confirmed that it is to launch its serviceconforming immediately to the CAI orCommon Air Interface which means userswill be able to register their phones with othertelepoint networks in 1991. Eventually it'shoped that all four telepoint operators willcooperate and provide transparent access forany user via any convenient public telepointbase station.

Ian Burley is the News and Features Editorfor BT Micronet, an on-line computer andtechnology magazine published on Prestelby British Telecom.

We shall have another Home -Basefeature for you next month.


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RADIO FEATURE

V ery high frequency (vhf) radio wasdeveloped during the second worldwar in order to communicate with

ships and planes. After the war, thisdevelopment was extended to the police andfire brigade in order to speed upcommunication and hence rescue.

A licence is required in order to transmitradio frequencies and the first private licenceswere granted to tugs in the Tyne and Wearrivers as well as to the Camtax taxi service inCambridge in 1947.

In 1947 the transmitted bandwidth was100kHz but today eight radio channels eachof 12.5kHz fit into this same bandwidth. Suchan economy is possible with the use of quartzcrystals to keep the transmitter from drifting,and other developments.

INCLUSIONS

Mobile radio can include any radiocommunication where the transmitter orreceiver or both are not fixed in location. Itincludes communication with aeroplanes(more strictly aeronautic radio) andcommunication with ships (more preciselymaritime radio). It also includes citizen bandradio and radio paging.

In this article, the popular concept ofmobile radio will be dealt with, ie a radiomounted in a vehicle travelling on land andcommunicating with a fixed base station.However, land mobile radio does not include

motorbikes are subjected to an even greaterlevel of vibration and dirt than those mountedin cars. The normal receiver output is 2W to4W but outputs as high as 10W can beprovided for noisy locations, eg fire enginepumps.

Handheld portables are of two kinds:handheld and bodywom. The handheld areusually one piece whereas the bodywom, asworn by the police foot patrols, incorporatethe tranceiver and battery in a case on thebody. The speaker, microphone, press to talkswitch and additional controls are worn on thelapel.

The Radio Regulatory ,ion in theDepartment of Trade a Industry isresponsible for frequency plan ig and is alsothe licensing authority in the LK. Under thewireless Telegraphy Act 1974 a licence isrequired to operate a private mobile radiosystem.

An additional licence under the TelegraphyAct 1984 is required for those offering aservice to others like radio telephony ormessage handling.

The mobile user must have the installationapproved and must not alter it withoutpermission once approved. The licenceconditions include limits on the effectiveradiated power of the transmitter and talkthrough must be under the control of the base

MOBILE RADIOjust vehicle mounted but handheld andtransportable as a packset as well.

A later article will deal with cellular radioand radio paging. The people who use mobileradio are: the police, ambulance, fire services,taxis and mini -cabs, couriers etc.

PRIVATE AND PUBLIC

The use of mobile radio for closed usergroups like police, ambulance, fire and thoseoperated by individual taxi and courierservices are called Private Mobile Radio(PMR). These employ a central controller torelay messages on jobs or locations to attend.The resulting saving in time, petrol and thecustomer satisfaction is well known by thoseoperating such schemes.

As opposed to these private networks thereare public networks like British Telecom'sSystem 4 Radiophone which is also connectedto the telephone network.

Private mobile users may share channels incongested areas and the authorities thatallocate the overlapping channels try toensure that the trades using these channels aredifferent so that business is not poached, andalso to spread the peak loading.

Mobile radio demands a no -handsoperation facility which is not usually

By Mike SandersNow you don't justlisten to the car

radio - you talk to it.satisfactory because of the ambient noise.This is overcome partly by mounting themicrophone on a boom, as in taxis. Formotorcycles the arrangement is easier sincethe earphones and microphone can be builtinto the helmet. For safety purposes the wirefrom the helmet to the tranceiver on themotorcycle is via a snatch connector. Thepress to talk button is usually on thehandlebars.

The demands on mobile radio equipmentare quite severe since the temperature in avehicle could be as low as -20°C at night andas high as +60°C in the daytime. The aerialcould be poorly sited and also pick upelectrical noise. There is also a good deal ofnoise from traffic, wind in a moving vehicleand other sources like a fire engine pump orconstruction sites. Mobile radios on

British Telecom kindly supplied thephotograph of one of their System 4Radiophone units.

station operator. Talk through is a simpleswitch connecting the base station transmitterto the base station receiver so that twomobiles can communicate directly with eachother. Alternately, talk through may becontrolled by tones from a mobile.

A fixed mobile may not be used as acontrol point and communications betweenfixed points is not allowed, except in anemergency. However point to point links areallowed for remote control of base stations.

OPERATINGPRINCIPLES

In order to assess the area of coverage anordnance survey map is used for a fieldstrength survey. The location and direction ofthe testing vehicle is recorded together withthe field strength of the signal both at baseand mobile for each location and direction.

Land mobile communication is in the vhfband (40-300MHz) and uhf (300-1000MHz).At the higher frequencies the signals travel instraight lines like light waves, therefore anyobstruction would cast a shadow. Toovercome this the aerial can be placed as highas possible but then produces interferencewith other users on the same frequency in adifferent area.


Tone frequencies in Hz

Digit ZVEI ZVEImodified

EEA/CCIR EIA

1 1060 970 1124 7412 1160 1060 1197 8823 1270 1160 1275 10234 1400 1270 1358 11645 1530 1400 1446 13056 1670 1530 1540 14467 1830 1670 1640 15878 2000 1830 1747 17289 2200 2000 1860 18690 2400 2200 1981 600

ZVEI Zentralverband der Electrotech-nischen IndustryEEA British Electronic EngineeringAssociationEIA British Electrical IndustriesAssociationCCIR Consultative Committee forInternational Radio

Fig.2 Mobile radio codes

The operator is usually remote from thetransmitting antenna and the usual method oflinking the two is by a two wire or four wireland line, as in Fig.1.

A mobile is allotted a five digit code andone of the systems, in Fig.2, ZVEI, OCIR,EIA, etc. chosen for transmission. The systemwill depend on the equipment manufacturer.

Keying a carrier on and off can also beused for digital transmission instead of theabove analogue tones. The disadvantage ofthe keying method is that it is not possible todistinguish whether a break in transmissionmeans 'spaces' or 'end of transmission'.

To overcome this, Frequency Shift Keying(fsk) or Fast Frequency Shift Keying (ffsk) isused and these methods are taking over fromthe codes in Fig.2. With fsk, the carrier isradiated to indicate mark and shifted to alower frequency to indicate space.

With ffsk a modulated carrier istransmitted with the modulation changingfrom mark to space. The modulatingfrequencies are 1200Hz for a binary 1 and1800Hz for binary 0.

With 12.5kHz channels it is generallyagreed that in terms of performance and cost,there is little difference between amplitudeand frequency modulation.

Speech contains most of its energy in theband 300Hz to 3000Hz and to squeeze evenmore channels into the spectrum, the top ofthe band is limited to 2700Hz. If the band istaken as nominally 3kHz then a carrier drift of2kHz is allowed plus a guard band making6.25kHz in all or 12.5kHz for doublesideband. Fig.3. illustrates how the spectrumis divided.

It may seem that the 2kHz allowable driftis wasteful on top of a 3kHZ speech band, butremember that this is only 0.015% of a150MHz carrier. It is hardly surprising thatquartz crystals are required to prevent thefrequencies from drifting.

In the receiver a signal of say 0.5uV willneed to be amplified some 140dB before it isof a sufficient level to be demodulated. At thecarrier frequency it is not practical to amplifyuhf or vhf signals to this extent and thenprovide enough selectivity to extract onespeech channel from all the others. Insuperheterodyne receivers (superhets forshort) this problem is overcome by convertingthe carrier to a lower frequency called anIntermediate Frequency (IF). A typical IF is10.7MHz.

AERIAL I

RECEIVE

AERIAL

V

TRANSMIT RECEIVE

1RADIO CONTROL

PANEL

I D1019011

VOTINGDECODER

AERIAL 3

V

RECEIVE

OPERATORCONTROL

PANEL

Fig 4. Voting

simultaneously could result in signalcancellation in some areas. An alternative isto transmit on each in turn, though this is timeconsuming. When the first option is chosenthe transmitters are carefully adjusted on thebasis of field strength measurements and,once adjusted, should not be touched.

Another choice is to space thetransmissions 7kHz apart on each aerial. Thismeans that on 25kHz vhf channels, the beatnote is outside the passband. However, with12.5kHz channels, the transmitter must eitherbe synchronised, or separated by a lowenough frequency, so that the beat note isoutside the audio range and filtered out.

METHODS OFOPERATION

The set up can be simplex or duplex. Inaddition, simplex can be subdivided into

Fig 3. Dividing up the spectrum.

6

CARRIER CARRIER CARRIERCARRIER CARRIER

Of L 2 AK \ 3 it...,;.,......, '%/ .N,'

----4.0.1o J D O

ICUG1900

o 3 5 6. 5 5 3 to 3 5 6.25 5 3 to 3 5 6-25kHz

AREAS OF POORCOVERAGE

In areas where the signal is weak the basestation could use several aerials spaced apartfor receiving only, as in Fig.4. Selecting thebest signal is called voting and out of speechband noise frequencies are used. The voltagefrom these frequencies controls the frequencyof a voltage controlled oscillator (vco) whichproduces a tone. The best tone, and henceaerial, may then be selected by the operator.To make the task easier, the tone may becoded into digits.

The above applies to reception at the basestation. For transmitting to the mobiles, thesame aerials spaced apart can be used.However, transmitting on all aerials

single and double frequency, and duplex canbe full duplex or semi -duplex.

Since frequency simple (Fig.5) uses thesame carrier frequency for transmitting aswell as receiving. Operators use such wordsas "over" or "Roger" to indicate to other usersthat they have finished speaking. Such asystem is adequate for low power applicationswhere it is also useful for all operators to heareach other. Such applications could be wherelocal hand portables are used, as in policecrowd control, tackling fires, largeconstruction sites, and so on.

For double frequency simplex, separatefrequencies are used for transmitting andreceiving, and while a station is transmitting,its receiver is switched off. The mobilestations hear only the base station and wouldnot be aware if any other mobile weretransmitting. See Fig.6.


RADIO FEATURE it)

13f

ti

MOBILE A BASE STATIONTRANSMITTING RECEIVING

101619021

MOBILE BRECEIVING

f 2

Tx Rx Tx Rx Tx

1Tx Rx Tx

f1

101619031

MOBILE ARECEIVING

BASE STATION MOBILE B MOBILE ATRANSMITTING RECEIVING RECEIVING

IC/161904j

12

JTx

BASE STATIONTRANSMITTING

RECEIVING

f2

MOBILE BTRANSMITTING

AND RECEIVING

Fig 5. Single frequency simplex. Fig 6. Double frequency simplex. Fig 7. Full duplex working.

In full duplex working a mobile canreceive and transmit at the same time, as witha telephone except that while a mobile istransmitting, its loudspeaker is switched off toprevent acoustic feedback. Reception is viaearphones in this instance. Fig.7 shows fullduplex working.

Full duplex working permits any mobile tointerrupt a conversation. In general, mobilesreceive only the base station, and the basetransmits pips when it is engaged.

Full duplex is more than what most mobileusers require and while it may be convenientto build earphones into a motorcycle helmet,most automobile users would not be happy towear an earphone. In the semi -duplexarrangement, the mobiles work simplex, whilethe base station works full duplex. This is asuitable compromise.

CRYSTALS, FILTERSAND SYNTHESISERS

Crystals play a major part in moderncommunications both in oscillators as well asfilters. Fig.8 shows the graph of temperatureagainst frequency variation of an AT cut ofcrystal. In Europe the temperature range ofinterest is -10°C to +55°C.

Fig 8. Frequency/temperatureresponse of AT crystal.Fig 9. Required frequency response offilter.

+60-

+40-

M +2o-

ceL5 8 0-` tv;"

c:- "2°-

.;.o.60

wnos

100-

90 -

BO.

70.

'41 ea

z 50-0

40

Li

30-

20-

10-

-60 -co -Zo 0 +io 40+io +eo +100+120TEMPERATURE IN CENTIGRADE

-25 -20 -15 -10 5 +10 +15 +20 +25kHz

CENTRE FREQUENCY 10.7MHz

When used as a filter, quartz is morecommonly used than ceramic filters. Suchfilters may consist of crystals wired togetherin lattice form to give the required response,illustrated in Fig.9.

Ceramic filters are made of piezo electricceramics with centre frequencies from a fewkilohertz to 10.7MHz and bandwidths of0.05% to 20% of the centre frequency. Crystalfilters have a high Q with the bandwidthrestricted to a few tenths of 1% of the centrefrequency, but this is sufficient for 4kHztelephony. Fig.10 shows a simple crystal filterwhere the thickness of the crystal is half thewavelength in the fundamental mode.

Fig 10. Simple crystal filter.

Until recently, a separate quartz crystalwas required to generate the transmit andreceive frequencies for each channel.Frequency synthesisers have the advantage ofusing a variable frequency oscillator wherethe frequency is controlled by a dc voltageand the circuit is then called a voltagecontrolled oscillator (vco).

A synthesiser operates by comparing thefrequency of the vco with the referencefrequency from a highly stable quartz crystaloscillator having an accuracy of better than +5parts per million. The output of the oscillatoris divided down by a programmable divider(programmable read only memory - prom) toa frequency equal to half the channelseparation.

This is 6.25kHz for 12.5kHz channels. Theoutput of the vco is also divided in half andapplied to a comparator together with thequartz oscillator output; shown in Fig.11. Adifference in frequency produces a dc voltagein the comparator output and this is used topull the vco frequency back on track.

But there is a slight problem. Withfrequency modulation, the carrier frequencyneeds to deviate according to the modulatingsignal but the comparator would keep pullingit back to synchronism. Therefore, what isrequired is two different reaction times, a fast

and a slow. The fast attack is required whenthe channel is changed or when the equipmentis first switched on. Then the slow reactiontakes over so that the audio can modulate thecarrier.

In simplex operation the transmitter andreceiver are not required at the same time,therefore, when switching between transmitand receive, the frequency of the vco can alsobe changed. But in duplex transmission twovcos are required.

Synthesisers usually have safety circuitsbuilt into them in case the vco is not on lockor a fault develops. This prevents thetransmitter from operating so that spurioussignals are not sent out.

SQUELCH

Even though frequencies are used atgeographic intervals, interference can stilloccur. This can be overcome by means ofsquelch circuits where the receiver remainsquiet in the presence of background garble butunmutes when a strong enough signal isreceived.

The noise in the receiver is used to mute itand the noise is swamped in the presence of astrong signal. To prevent the receiver beingshut down by frequencies in the speech band,a voltage derived from the signal is used tohold the receive path open.

Squelch circuits can be used which biasthe first audio stage to cut off until a strongenough signal is received and the stage isswitched on with the assistance of theAutomatic Gain Control (agc). This helps toremove background noise in channels sharedby multi -users such as citizen's band radio.

Fig.12 shows one kind of squelch circuitwhose operation depends on the setting of thesquelch control and agc voltage. With thesquelch off (base of Q2 earthed), Q3 is

Fig 11. Simple synthesiser.


RFIN

FROMAOC

SQUELCHRANGE

FULL ON

SQUELCHCONTROL

OFF

01RF AMP

+VE

aP 024 -%I,

SQUELCH AMP

Fig 12. Squelch circuit.

03

AF AMP

switched on. But with the squelch on, Q3 iscut off and the agc must go more negative toswitch Q2 off and allow Q3 on again.

Another method of unmuting the receiveris to send a signal containing a speciallycoded tone. Since there are so many differenttransmitting standards the ElectronicIndustries Association (EIA) has agreed anumber of standard tones.

The tone is called a Continuous ToneCoded Squelch System (ctcss) or tone lock. InAustralia it has been compulsory since 1984and is called Quiet Base Station (qb).

The use of ctcss is particularly importantwhen the talk -through switch is thrown so thatinterference is not relayed to the next link.Incidentally ctcss does not eliminate theinterference, it only suppresses it; the garble isstill there. The ctcss tones are sub audiobetween 67Hz and 250Hz and transmitted at alow level, below the signal level, to preventinterference with the signal.

One final point connected with noisesuppression is that a mobile could betransmitting background noise if the mobileuser forgets to switch off his transmitter. Toprevent this, transmission time-out timers areused and the user has to press his 'press -to -talk' button at regular intervals.

CONTROL EQUIPMENT

The transmitted powers can be as low as3W at vhf in Europe. In the UK thetransmitted power is higher, IOW to 25W atvhf and from 5W at uhf. For mobile radio themaximum permitted radiation is 25W but canbe as high as 100W for the police and firebrigade. These output powers apply to bothamplitude modulation and frequencymodulation.

The base station transmitter usescompressors to make loud sounds softer andto also raise the average level of the signal.Sharp peaks could still exist after thecompressor, and a clipper usually follows it to

ensure that signals of a high level do not geto the modulator. Any high level signalswould cause overmodulation and hencedistortion. But a clipper is a non-linear deviceand spurious frequencies are produced when asignal exceeds the level set by it.Consequently, a low pass filter is includedafter the clipper to exclude the spuriousfrequencies. Sometimes a fault could developon the aerial which could damage thetransmitter. Therefore control circuits areincluded, which measure the standing waveratio, which is the ratio of forward power toreflected power. If a fault develops, thetransmitter can be shut down. Base stationtransceivers do not have to put up with thevibration, dirt, moisture and temperaturevariations of transceivers mounted in vehicles.

CALL STACKING

When the channel occupancy is low, forinstance for security guards checking out aperimeter fence, there is no reason why alloperators should not hear each other and thereis little point in complicating the set up withselective calling.

However, when the channel occupancy ishigh, selective calling is not only necessarybut ways must be found of speeding up itsprocedure. One method of speeding up theprocess is to acknowledge a callautomatically. This can be done by a mobiletransmitting its call sign. Additionally, the callsign can be transmitted in an emergency if theequipment is fitted with an alarm button. Thecall sign can also be used when the mobilepresses its bid button to place it in the queuefor speaking to the base station.

This is another area that can be automated.For a large pick-up and delivery fleet coveringshort distances the base station will want astatus up -date, for instance location anderrand status. Such an update is semi-automatic since the mobile user has toprogramme his transmitter. However, the

transmission can be automatic when the basestation interrogates it or when the mobiletransmits next.

At the base station, the calling mobiles canbe displayed in a queue on a vdu, by call sign.The base station operator can, if he wishes,select the calls out of turn, and emergencycalls can be arranged to reach the top of thestack or with a flashing sign on the vdu.

If the mobile fleet is large only the top 15mobiles, say, may be displayed on the screenwith the rest held in a queue in the memory.This also leaves screen space for otherinformation.

SYSTEM 4RADIOPHONE

British Telecom operates the System 4automatic radiophone service. Under theWireless Telegraphy Act of 1949, BritishTelecom is permitted to make transmissionsbetween mobiles and base stations, andoperates 110 channels in the 160MHz band.

These 110 channels in the vhf band arespaced at 12.5kHz. About 10,000 customersuse the service which has good coverage ofrural areas, unlike the cellular systems whichare primarily aimed at high density urbanareas.

The first UK radiophone system was set upin South Lancashire in 1959 and the Londonservice in 1965. This manual system wascalled System 1 and served 320 customers inLondon.

A manual service, called System 3, using37 channels was set up in 1971 and increasedto 55 channels in 1973. More than 3000mobiles in the London area used the servicewhich was more than half the national total(with the Midlands second in demand).

These were half duplex channels and sincethe Home Office would not make furthercapacity available it was decided to reduce thechannel spacing to 12.5kHz on this alreadyheavily congested service. This increased thechannels to 110 and System 3 closed at theend of 1986. However, the pressure ofdemand in London was satisfied only by thecellular service, which will be dealt withseparately.

System 4 is based on the MobileAutomatic Telephone System (MATS -B)developed by TeKaDe which is now part ofPhilips Kommunikations Industrie AG ofWest Germany. There are some half a dozenMATS -B systems around the world alloperating a 25kHz channel spacing. BritishTelecom is the only one to operate the systemwith 12.5kHz channel spacing.

Also the West German system is based oncities whereas System 4 is based on sevenindependent regions, as in Fig.13. Each regionhas its exchanges and each exchange has basestations. The TeKaDe exchanges are calledRadiophone Control Exchanges (RCE) andeach exchange has a maximum capacity of 42base stations and 96 radio channels.

The base stations have an EffectiveRadiated Power (erp) of 25W and can providecommunication up to 55km. However, the


Fig 13. System 4 regional coverage.

needs of customers and the geography of theland usually means that the distance is limitedto about 20km.

The mobile transceivers themselvesconsume as much as 3.5A at 12V so they arenot exactly portable. A mobile user canoverride the automatic selection procedureand choose a base station he knows is nearby.Each base station transmits an Idle MarkingSignal (IMS) on all the unoccupied channelsand there are seven different IMS codeswithin each area so that neighbouring basestations can be identified.

The mobile user enters an IMS code toselect his base station and hence the channel.In practice this is too complex for most usersand the normal process for setting up a call isfor the mobile to scan all the channels. If thereis more than one free channel, the mobileselects the one with the strongest signal. Thescan takes about 2 seconds and some mobileequipments are able to scan the channelstwice searching for a suitable link.

Users have to dial the full national numbereven though the called party may be on thelocal exchange. This number is transmitted tothe base station which transmits it back to themobile on the other half of the duplex link.

The mobile then carries out a comparisonwith the dialled digits. If these areincompatible, the information has beencorrupted and the mobile tries again. If thetransmission has been successful the basestation checks with its RCE that the mobile ispermitted to use the zone.

For calls to a mobile, the caller needs to

know the area code in which the mobileoperates since there is no arrangement forrelaying calls from one region to another.Most mobile users choose one or two zones,therefore callers usually know the area code.

The RCE chooses a base station fortransmitting the call and pages it. One of the110 channels is reserved for this purpose andthe base stations are divided into seven groupswith different paging signals to minimiseinterference and confusion.

In turn, the mobiles tune into the pagingchannel automatically when not making orreceiving a call. The mobile listens for itsnumber and sounds the bell. If the mobile userdoes not answer within one minute, the call isabandoned to avoid tying up the link andequipment. No charge is made to the caller forabandoned calls.

Calls can be received from anywhere in theUK and for outgoing calls, even internationaldirect dialling is possible. The System 4service is available on Sealink ferries andeven on some gas production platforms andsupport vessels near the coast.

RADIO EMISSIONDESIGNATION

The International TelecommunicationsUnion (ITU) has agreed a code to describeradio emission. The code is alphanumeric andnine symbols long. It is made up as follows:

Symbol 1 to 4 bandwidth5 modulation type6 type of modulating signal7 information type for transmission8 signal details9 type of multiplexing

Symbol 1 to 4 : bandwidth

The bandwidth is defined by one characterand three digits, for example:

800Hz is 800H25kHz is 25K0I 2.5kHz is 12K5

Symbol 5 : modulation type

N = emission of a carrier withoutmodulation.

AMPLITUDE MODULATION

A = double sidebandH = single sideband, full carrierR = single sideband, reduced or variable

carrier levelJ = single sideband, suppressed carrierB = two independent sidebandsC = vestigial sideboard

ANGULAR MODULATION

F = frequency modulationG = phase modulationD = amplitude and angular modulation

either simultaneously or in a prearrangedsequence

RADIO FEATURES

PULSE MODULATION

P = train of unmodulated pulsesK = train of pulses modulated by

amplitudeL = train of pulses modulated by widthM = train of pulses modulated by positionQ = a train of pulses where the carrier is

angle modulated during the pulse periodV = a combination of the aboveW = modulation not covered above, where

the carrier is modulated, either simultaneouslyor in a prearranged manner, in a combinationof two or more of the methods; amplitude,pulse, angle

X = methods not covered above

Note: the above methods involve themodulation of a pulse train by the modulatingsignal but where a carrier is directlymodulated by a pulse train it must bedesignated under angular or amplitudemodulation.

Symbol 6 : type of modulating signal

0 = no modulating signal1 = signal channel with digital information

without a modulating sub -carrier2 = as 1 but using a modulating sub -carrier3 = signal channel containing analogue

information7 = two or more channels with digital

information8 = two or more channels with analogue

information9 = a mixed system where one or more

channels contain analogue information andone or more channels contain digitalinformation

X = cases not covered above

Symbol 7 : information type fortransmission

N = no informationA = telegraphy for audio receptionB = telegraphy for automatic receptionC = facsimileD = data transmission, eg telemetryE = telephony including sound broadcastF = televisionW = a combination of the aboveX = cases not covered above

Symbol 8 : signal details

A = a two condition code with elements ofdifferent numbers and/or durations

B = a two condition code with elements ofthe same number and duration without errorcorrection

C = as for B but with error correctionD = a four condition code in which each

condition represents a signal element of oneor more bits

E = a multi condition code in which eachcondition represents a signal ele,nent of oneor more bits

F = a multi condition code in which eachcondition or combination of conditionsrepresents a character


G = monophonic sound of broadcastquality

H .= stereophonic or quadraphonic soundof broadcast quality

J = sound of commercial quality excludingcategories K and L

K = sound of commercial quality involvingfrequency inversion or band splitting

L = sound of commercial quality withseparate frequency modulated signals to

control the level of demodulated signalM = monochrome signalN = colour signalW = a combination of the aboveX = cases not covered above

Symbol 9: type of multiplexing

N = no multiplexingC = code division multiplexF = Frequency Division Multiplex (fdm)T = Time Division Multiplex (tdm)W = combination of fdm and tdmX = multiplexing not covered above

A mobile radio transmission of 25kHzwould be described as 25K0 F3EJNindicating that it is frequency modulated witha single channel containing analogueinformation. The analogue information is

telephony of commercial quality and nomultiplexing is involved.

CONCLUSIONS

This article has traced the early history ofmobile radio and shown how eight 12.5kHzchannels now occupy the 100kHz bandwidthformerly occupied by only one channel. Landmobile radio has been concentrated on, to keep thecoverage simple.

Even so, land mobile covers not only vehiclemounted but hand held and transportable packsetsas well. The growth of private mobile radio wasnoted and the severe operating conditions of dust,vibration and temperature were covered.

Any operator wishing to set himself up as amobile operator must note the licensing conditions.

Various 5 -code systems of transmission areused together with fsk and ffsk. Although guardbands are used to separate transmission, quartzfilters are required to keep the frequencies on track.Areas of poor coverage use several antennas whichneed to be carefully adjusted to prevent thetransmitted signal from cancelling. Operatingmethods using both simplex and duplex were dealtwith.

The principles of vcos in frequency synthesiserswere outlined as well as the use of squelch insubduing background noises. For an operator witha large fleet of vehicles some form of call stackingcan simplify the task.

The growth of the System 4 automaticradiophone was then described as a form of publicmobile radio followed by the ITU agreed radioemission designations.

The implementation of cellular radio in citieshas supplemented rather than displayed landmobile radio and will be described separately inanother article.

It is hardly surprising that a society which reliesso heavily on the telephone for communicationsexpects the next development, that of being able tocommunicate while on the move. No doubt thisexpectation will continue to grow putting pressureon the radio spectrum as well as theresourcefulness of planners.

There is no doubt that the future will see the useof satellites for mobile radio as well as theexpectation of the individual to communicate byradio with anyone even while outside his owncountry. And that's a tall order. (But an order thatINMARSAT are intent on fulfilling. Ed).

GLOSSARY

AGC automatic gain controlCTCSS continuous tone coded squelchsystemERP effective radiated powerFFSK fast frequency shift keyingFSK frequency shift keyingIMS idle marking signalMATS mobile automatic telephone systemPMR private mobile radioPROM programmable read only memoryQB quiet baseRCE radiophone control exchangeVCO voltage controlled oscillator

Reltztaetriue CE 8C

ile4nNale:4 ELECTRONICS ,)

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Your Ed looks at some of the new books recently received.

A Concise Introduction to GEM. J.W.Penfold. Babani.£2.95. BP230. ISBN 0-85934-175-5:GEM is a software system written to make computers easier to use. Thesystem uses pictorial representations, usually known as icons, to visuallydisplay disc file names and types on the screen. The icons are accessedand routines selected by using a 'mouse' to control a pointer on thescreen. The book has been written as an introduction for new users, and isstructured to provide a convenient, compact source of reference for moreexperienced users. It can also act as a quick 'refresher' for anyone whohas not used GEM for some time. The book explains the nature of GEMand what it can and cannot do. It explains how to use GEM for routinehousekeeping of disc files, catering for both hard and floppy disc users.You are told how to configure and run applications from the GEMdesktop to best advantage, including menus, graphics and other specialfeatures available to users from within the applications programs. Allversions of GEM up to release 3.01 are covered, including versionssupplied with the Amstrad PC and Atari ST computers. This is a bookwhich usefully supplements the manual supplied with the GEM system.

A Concise Introduction to OS/2. N. Kantaris. Babani. £2.95.BP260. ISBN 0-85934-205-0.OS/2 is a new computer operating system. Users of PC/MS-DOS willimmediately detect a certain familiarity with OS/2 as a great deal of thisnew system is based on it. This book does not seek to replace thedocumentation you receive with the OS/2 operating system, but only tosupplement and explain it. The book covers both command lineprocessing and OS/2's Presentation Manager which should be somewhatfamiliar to users of DOS version 4.0 as the menus employed in the DOSshell of that version are consistent with those used in Microsoft Windowsand hence similar to those used in OS/2. If you are a multitasking PC userand you want to get the most out of your computer in efficiency andproductivity, then you really ought to learn OS/2. This book will help youdo just that since it was written with the non -expert, busy person in mind.As such it has an underlying structure based on "what you need to knowfirst, appears first". The book covers both the command -line mode ofprocessing and the Presentation Manager of OS/2 Standard Edition 1.1 asimplemented by IBM and Microsoft.

A Concise Introduction to Lotus 1.2.3. N. Kantaris.Babani. £2.95. BP261. ISBN 0-85934-206-9.Lotus l-2-3 is an integrated software package containing three majortypes of applications; spreadsheets, graphics and data management. Thepackage is operated by selecting commands from menus or by writingspecial macros which utilise the Lotus Command Language to chaintogether menu commands. If you are a PC user and want to get to gripswith Lotus 1-2-3, then this book will teach you how in a short andeffective way. The book explains how the software can be used to buildup simple spreadsheet examples, edit them, save them and retrieve them.You are told how to format labels, enter and format numbers, change thedefault width of cells, enter formulae and Lotus 1-2-3's built-in functions,and print a work sheet. It goes on to explain how titles can be frozen onscreen, and how to add graphs to a worksheet, add legends and titles tographs, view and print graphs. Explanations are given about setting adatabase management system, sorting and searching a database, using thefind and extract commands to query information held in a database. Thecreation, use and debugging of simple macros is explained, as is thecreation and use of range names. The book lists all Lotus 1-2-3 indicators,functions and macro commands so that it is self contained and can beused as a reference book long after the reader has become an expert in theuse of the program.

A Concise Introduction to WordPerfect. R.A.Penfold.Babani. £2.95. BP262. ISBN 0-85934-207-7.Wordperfect has now become the most popular word processor packagefor the IBM PC range and the numerous compatibles in use around theworld. It is also available in versions for many other computers, includingthe Atari ST series and the Commodore Amiga range. This book is writtenfor beginners to word processing, and assumes a minimal amount ofknowledge about using computers and running applications programs. Itexplains how to enter and edit text, and quickly move around largedocuments. Performing block operations is covered, including move,copy, delete, case changing and printing. Text formatting ready for printingis explained, including the use of advanced features such as newspapercolumns and graphics, as well as basic formatting , such as line spacingand setting margins. You are also told how to install printers and print outdocuments using various fonts and printing effects. The use of mail -mergefacilities to customise standard letters in explained, and you are shownhow exploit useful facilities such as the spell -checker, thesaurus and sortroutine. The book covers up to Wordperfect version 5.0.

A Concise Introduction to dBASE. J.W.Penfold. Babani.£3.95. BP263. ISBN 0-85934-208-5.The software package dBASE is one of the few database systems whichcan truly said to be 'industry standard'. This family of database programsdates back to CP/M computers with dBASE II, went on to the IBM PCfamily with dBASE III and III Plus, and continues with the recentlyintroduced dBASE IV. This book covers all versions up to dBASE IVrelease 1.0. The author says that the purpose of this book is not to teachyou all there is to know about databases and programming dBASE.Indeed, he doubts that any one book could do that. Neither is it claimedthat this book describes all the dBASE functions and commands. Rather, itis designed to provide the information you need to get started in a conciseform, and to provide a convenient source of reference for the future on themost commonly used dBASE features. The book explains what a databaseis and does, how to create and use a database with dBASE, how to writesimple dBASE programs to help you with your work, and what thedifferences are between the various versions of dBASE.

Learning to Program in C. N. Kantaris. Babani. £4.95.BP258. ISBN 0-85934-203-4.C is a highly portable general purpose programming language which isstructured, modular and compiled. It is a language designed to be fasterand more efficient than Basic and yet simpler to write than Assembler. Assuch it is by far the most up and coming language of today. The book is aguide to C programming, first of all introducing C statements andexplaining them with the help of simple but completely workingprograms. Graded problems are set at the end of each chapter, some withfinancial or scientific bend, so that users can choose their own level ofproblem difficulty on which to practice with some additional choice in thepreference of field of application., Full working solutions appear at theback of the book. Chapters 1-3 deal with the basic C statements whichcontrol program flow and allow the user to manage most aspects of thelanguage. Chapters 4-5 introduce the concepts of string arrays, numericarrays and function subprograms which expand the programmingcapabilities of the user beyond the beginner's level. Chapter 6 dealsentirely with the data -file handling on disc, while Chapter 7 deals withunique C structures, both of which should be of interest to all those whoneed to process large quantities of data.

From Atoms to Amperes. F.A. Wilson. Babani. £3.50. BP254.ISBN 0-85934-199-2.The author says that he has written this book for anybody with a tinge ofcuriosity, and he has made it easy reading even for those who have nevergot on well with the subject of physics, or who have forgotten all theyever knew! He has deliberately kept low the level of mathematicsrequired, though a moderate knowledge of school algebra and a little basictrigonometry is preferable, The illustrative calculations used can readilybe made using a pocket calculator. The book explains in crystal clearterms the absolute fundamentals behind electricity and electronics. Itshould really help you to discover and understand the subject, perhaps forthe first time ever. I heartily recommend this book to all beginners, and tothose looking for a simple memory refresher.

PUBLISHER'S ADDRESSBernard Babani (Publishing) Ltd, The Grampians, Shepherds Bush Road,London W6 7NF.


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hile it is true that advances indevice technology have producedmuch better triacs, thyristors are

still more robust, and they offer lower voltagedrop when switched on. nevertheless, for lowand medium load currents triacs offer asimpler and more economical solution. Itshould be noted, however, that such devicesas high current solid state relays useantiparallel thyristors for load switching.

Phase angle speed control of a motor canbe accomplished by means of phase cuttingcontrol, like a lamp dimmer. However,because motor loads normally have a laggingpower factor (more inductance than resistancewhen the motor is stationary) the phasereference must be taken from the live of themains directly rather than through the load. Acircuit based on the lamp dimmer principlewould receive a false phase reference, andperhaps fail to trigger or perhaps trigger onhalf cycles only, because the current is out ofphase with the voltage. With an inductiveload, when the triac switches off at the zerocurrent point, this is not near to the zerovoltage crossing of the mains. Consequently,the voltage across the triac rises rapidlytowards the mains voltage, slowed only by thetime constant of the inductor and whatevercapacitance is present across the triac.

This effect can falsely trigger the triac, butmore often it simply confuses simple phasecontrol cicuitry into malfunctioning.

RAMP GENERATOR

The design shown in Fig.1 takes accountof this problem and draws its phase referencefrom the mains live, via R1 and the opto-isolator IC2. The phototransistor in the opto-isolator conducts and is saturated except at orvery near to the mains zero crossings, when itswitches off, allowing TR I to switch on andreset C3.

AC MOTORCONTROL

This month Andrew Armstrong answers the questionfrom D.L Jones of Mid Glamorgan:

Can you supply me with a design for a variable speed control unit for AC series woundmotorg in the I -2kW range (for electric tools)?. 1 have a copy of PE October 1987, in whichthere is a design of a thyristor controlled unit. 1 am sure that, with the advance of electronics, abetter design is possible.

After being reset, C3 charges again via R3,to approximately 6.5V. The way in which thisvoltage is calculated is illustrated in Fig.2.The ramp is exponential rather than linear, butthe part of the exponential function used isclose enough to linear for the purposes of thisdesign.

The ramp is buffered by IC3A and fed tothe IC3B, which is wired as a comparator.When the ramp voltage exceeds the voltageon the inverting input, the comparatorswitches on and triggers the triac opto-isolator. This in turn triggers the output triac.The point on the ramp which the comparatorswitches is determined partly by the setting ofVR1, the speed control, and partly by thefeedback signal taken from the output via IC4.Thus the triac is triggered on part -waythrough each mains half -cycle, and is re -

triggered at the same fraction through the nextmains half -cycle regardless of the precisetiming of the zero current point.

The circuit would work as a simple motor -speed controller without the feedbackcircuitry (from R12 to R8). This extracircuitry is intended to stabilise the speed ofthe motor to some extent, by detecting theincrease in current as the motor slows. Part of

the impedance of the motor is due to theresistance of the wire, and part due to theinductance. However, when the motor isrunning, a major part of the motor's effectiveimpedance is due to the back emf of themotor. The reduction of this part of themotor's impedance, and the resulting increasein current, is used to increase the phase angleof the motor drive, supplying it with morepower.

This is a positive feedback action, and iftoo much feedback of this nature is applied,the motor speed will tend to oscillate or runaway. The correct amount of feedback,adjustable by means of VR2, will simplyreduce the tendency for the motor to slowdown if its load is increased.

This feedback circuit has a limited rate ofresponse, because it is working on an acwaveform, and adjusting a triggering phaseangle as its response. Its time constant is setby C5 and R11, and may require someexperiment to optimise for the motor in use.The value of R12 controls the maximumamount of feedback available. Ideally, itsvalue should be chosen so that the bridge andthe led in the opto-isolator are just barelyforward -biased at the normal operating

Fig 1.

R1

1001,1W

1C3 - LM358 OR 1/2 LM321.

s, 55 TURNS OF 1mm ENAMELLEDCOPPER WIRE ON A T94-40TORROIDAL CORE I C IRKI T1

1

dik

IC2

L_

R 2

4.7k

R3

180k

C3om 100n

R.

2k2

V8113k

S PEED

+12V

R6

100kR9 1M

-V\A-U. 1n

88

IC3b

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ok

VR2

10k

0 6808 - - -

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105

MOC 30 21 I 1L-- -J-7

1 C5+=[.7p

_J

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Rs10011

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Fig 2. SWITCHED ON AT t. 0

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Fig 3.

current of the motor. Increases in motorcurrent will then put more current through theopto-isolator, and apply more feedback signal.If the value of R12 is excessive the mainproblem will be that of too much heat

dissipation.

OUTPUT STAGE

A high current triac is used to control theoutput. A TXAL2215B is shown in thisexample. It was chosen because it is an

isolated tab triac rated at 400V, I5A. A similartype of triac would be equally suitable and ifthe heat sink is completely enclosed in anearthed or insulated case such that there is nopossible accesss for people's fingers , etc, thenan isolated tab triac is not needed.

A certain amount of output filtering is

shown in this circuit, because a triac switchingnear the peak of the mains half -cycle cantransmit a lot of interference down the mains

wiring. LI and C7 effectively form a low-passfilter to limit this interference. C6 and R13form a snubber network to protect the triacfrom spikes, and rapid changes of voltagecaused by switching an inductive load. Thecomponent values shown are for example only,and may need adjustment in the light of theactual load to be used.

The only remaining part of this design isthe power supply, Fig. 3. A conventionaltransformer, rectifier, capacitor and regulatordesign is used to provide a clean I2V supplyfor the circuit. All the control circuitry is

isolated from the mains voltages, and is thusable to be controlled by external signals ifnecessary. The design as shown givesmaximum power when the wiper of VR1 is atzero volts, and zero power when it is at themost positive end of the pot.

If an externally derived positive goingcontrol signal is to be used, an LM324 opampcould be used instead of the LM 358, and oneof the remaining parts of the opamp could be

used to invert and buffer this signal.The speed control offered by this circuit is

of limited efficiency, and if really accuratespeed control is desired, some sort of shaftencoder should be used to derive a speedrelated signal. This can then be summed withthe basic speed control signal in extra circuitry,using other opamp parts.

This precise circuit has not been built andtested, though all parts of it have been usedsuccessfully in the ways indicated.

ABOUT ASK PEAsk PE is a most -monthly column in which

the most interesting readers' technical enquiries(in the opinion of the Editor) will be answeredto the best of the colunist's ability. Individualqueries will not be answered, even if stampedaddressed envelopes are sent. Please markenvelopes clearly "ASK PE", and enclose noother correspondence because these envelopeswill be forwarded straight to the columnist. PE

Free demonstration discsBorland C++ Procyon Lisp A.I.algorithms Hashing routines in C

Programming the X.Client

In the UK's only news standmagazine for developers - £1.50

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Distortion Analyser Model P40 £64.95Analyse at 30, 100, 300, lk, 3k and 10k Hz

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A F Millivoltmeter Model P41 £64.95Wide range: 10 Hz to 2 MHz, and 1 mV to 100 V

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Output cable: add £3.50. PP9 battery: add £2.90 inc VATCOLEBOURN ELECTRONICS, Dept. PE2, 20 FollyLane, St. Albans, Herts, ALM3 5JT. Tel: 0727 44785


BAKERS DOZEN PACKSAll packs are £1 each Note the figure an the exteme left of thepack ref number and the next figure is the quantity of items in thepack, finally a short description.

BD2

809

BD11

BD13

BD22BD30

BD32

BD42

BD45

BD49

BD56

BD67

BD103A

BD120

BD128

BD132

BD134

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BD148

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BD180BD193

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BD305BD653

5 13A spurs provide a fused outlet to a ring main wheredevices such as a dock must not be switched off.2 6V IA mains transformers upright mourning with fixingdamps.I 6.5 -in speaker cabinet ideal for extensions, takes our

speaker. Ref. BD137.12 30 watt reed switches, it's surprising what you can makewith these - burglar alarms, secret switches, relay, etc., etc.2 25 watt loudspeaker two unit crossovers.2 Nicad constant current chargers adapt to charge almostany nicad battery.2 Humidity switches, as the air becomes damper themembrane stretches and operates a miaoswitch.5 13A rocker switch three tags so on/off, or change over withcents off.1 24hr time switch, ex -Electricity Board, automatically adjustfor lengthening and shortening day, original cost £40 each.5 Necn valves, with series resistor, these make good nightlights.1 Mini uniselector, one use is for an electric jigsaw puzzle,we give circuit diagram for this. One pulse into motor, movesswitch through one pole.1 Suck or blow operated pressure switch, or it can beoperated by any low pressure variation such as water level inwater tanks.1 6V 750mA power supply, nicely cased with mains inputand 6V output leads.2 Stripper boards, each contains a 400V 2A bridge rectifierand 14 other diodes and rectifiers as well as dozens ofcondensers, etc.10 Very fine drills for pcb boards etc. Normal cost about 80peach.2 Plastic boxes approx 3in cube with square hole through topso ideal for interrupted beam switch.10 Motors fa model aeroplanes, spin to start so needs noswitch.6 Microphone inserts - magnetic 400 ohm also act asspeakers.4 Reed relay kits, you get 16 reed switches and 4 coil setswith notes on making Go relays and other gadgets.6 Safety cover for 13A sockets- prevent those inquisitivelittle fingers getting nasty shocks.6 Neon indicates in panel mounting holders with lens.6 5 amp 3 pin flush mounting sockets make a low cost discopanel.1 Mains solenoid, very powerful, has tin pull or could push ifmodified.8 Keyboard switches - made for computers but have manyother applications.1 Oectric dock, mains operated, put this in a box and youneed never be late.5 12V alarms, make a noise about as loud as a car horn.Slightly soled but OK.1 Panostat, controls output of boiling ring trap simmer up

50 Leads with push -on 1/4in tags - a must for hock -ups -mains connections etc.2 Oblong push switches for bell a chimes, these can mainsup to 5 amps so could be foot switch if fitted into pattress.1 Mini 1 watt amp for record player. Will also change speedof record player motor.3 Mild steel boxes approx 3in x 3in x tin deep - standardelectrical.1 Tubular dynamic mic with optional table rest2 Miniature driver transformers. Ref. LT44, 20k to 1k centretapped.

BD548 2 3.5V relays each with 2 pairs changeover contacts.BD667 2 4.7 ul nal-polarised block capacitors, pcb mooning.There are over 1,000 items in our Bakers Dozen List If you want aborn** copy please request this when ordering.TOASTERS 2 SLICE toasters - may need slight attention only £3 each ref3P84.GBGER COUNTER KIT includes tube, PCB and all components to makea 9V counter £34. Ref 39p1PERSONAL STEREOS Again customer returns but complete and withstereo head phones a bargain at ally £3.00 each. Ow ref 3P83.MAINS OPERATED MICROWAVE CONTROL PANEL with Touchswitches. This unit has a 4 digit display with a built in dock and 2 relayouMuts - one for power and 1 for pulsed power level. Could be used faall sorts of timer contra applications. Only £6.00. Our ref 6P18.EQUIPMENT WALL MOUNT Multiangle for speakers etc. £3 each ref3P72.SUB -MIN TOGGLE SWITCH Body size 8mm x 4mm x 7mm SBDT withchrome dolly sizing nuts. 3 for £1. Order ref BD649.COPPER CLAD PANEL fa making PCB. Size approx 12in long x 8.5inwide. Double -sided on fibreglass middle which is quite thick (about1/15n) so this world support quite heavy components and could evenform a chassis to hold a mains transformer, etc. Price £1 each. Our refBD683.MAINS FANS Brand new, snail type. Approx. V z e approx. 70W airy£4.00. Ref. 4P58.REAL POWER AMPLIFIER for your car, it has 150 watts cutout.Frequency response 20hz to 20Khz and signal to noise ratio better than60dB. Has built in short circuit protection and adjustable input level to suityour existing car stereo, so needs no pre -amp. Works into speakers ref.30P7 described below. A real bargain at only £57.50. Order ref: 57P1.

POWERFUL IONISERGenerates approx. 10 times more IONS than the ET1 and similarcircuits. Will refresh your home, office, workroom etc. Makes you feelbetter and work harder - a complete mains operated kit, case induded£12.50 + £2 p&p. Our ref 12P5/1.

REAL POWER CAR SPEAKERS. Stereo pair output 100W each. 4 -Ohmimpedence and consisting of 6.5in woofer, tin mid range and tin tweeter.Ideal to work with the amplifier described above. Price per pair £30.00.Order ref: 30P7.STEREO CAR SPEAKERS. Not quite so powerful -70w per channel. 3inwoofer, tin mid range and lin tweeter. Price per pair: £28.00. Order ref:28P1.VIDEO TAPES These are three hour tapes of superior quality, madeunder dunce tom the famous JVC Company. Offered at only £3 each.Our ref 3P63. Or 5 for £11. Our ref 11P3. Or for he really big user 10 for£20. Our ref 20P20.

ELECTRONIC SPACESHIP. Soundand impact controlled, responds toclaps and shouts and reverses when ithits anything. Kits with really detailedinstructions. Ideal present for buddingyoug electrician. A youngster shouldbe able to assemble but you mayhave to help wiht the soldering of thecomponents on the pcb. Complete kit£10. Our rel. 10P81.

12" HIGH RESOLUTION MONITOR Amber screen, beautifullycased for free standing, needs only a 12v 1.5 amp supply.Technical data is on its way but we understand these are TTLinput. Brand new in makers' cartons. Price: £22.00Order ref 22P2.

14" COLOUR MONITOR made by the American Display Tek Company.Uses high resolution tube made by the famous Japanese Toshiba company.Beautifully made unit intended for console mounting, but top and sidesadequately covered by plated metal panels. Full technical spec, on its wayto us. We have a limited number of these. AU brand new still in maker'scartons. Price: £89 each plus £6 insured carriage. Order ref 89P/1.COMPOSITE VIDEO KITS. These convert composite video into separateH sync, Vsync and video. Price £8.00. Our ref 8P39.BUSH RADIO MID SPEAKERS Stereo pair. BASS reflex system, usinga full range 4in driver of 4 ohms impedance. Mounted in very nicely madeblack fronted walnut finish cabinets. Cabinet size approx 8.5in wide, 14inhigh and 3.5in deep. Fitted with a good length of speaker flex andterminating with a normal audio plug. Price £5 the pair plus £1 past Ourref 5P141.3.5in FLOPPY DRIVES We still have two models in stock: Single sided,80 track. by Chinon. This is in the manufacturers metal case with leadsand IDC cainectas. Price £40, reference 40P1. Also a double sided, 80track, by NEC. This is encased. Price £59.50, reference 60P2. Both arebrand new. Insured delivery £3 on each or both.REMOTE CONTROL FOR YOUR COMPUTER With this outfit you canbe as much as 20 feet away as you wit have a joystick that can tramsmitand a receiver to plug into and operate your computer and TV. This isalso just right if you want to use it with a big screen TV. The joystick hastwo fire buttons and is of a really superior quality, with four suction cupsfor additional control and one handed play. Price £15 for the radiocontrolled pair. Our ref I 5P27.ASTEC PSU. Mains operated switch mode, so very compact Outputs+12v 2.5A, +5v 6A, +-5v .5A, +-12v 5A. Size: 7.5n long x 4.75'n wide x225in high. Cased ready fa use. Brand new. Normal price £30v, ourprice only £12.95. Order ref I3P2.VERY POWERFUL 12 VOLT MOTORS. 1/3rd Horsepower. Made to drivethe Sindair C5 electric car but adaptable to power a go-kart, a mower, a railcar, model railway, etc. Brand new. Price E20 + £2 postage. Our ref. 20P22.ELECTRONIC SPEED CONTROL KIT Suitable for controlling ourpowerful 12v motors. Price £17.00. Ref. 17P3 (heatsink required).

SINCLAIR C5 WHEELS. Set of 4 wheels with inner tubes and tyres.213' dia. halt wheels and 2 6' dia. rear wheels. Brand new £24 set.Ref 24P3.

PHILIPS LASERThis is helium -neon and has a power rating of 2mW. Completely safeas long as you do not look directly into the beam when eye damagecould result. Brand new, fug spec. £35 plus £3 insured delivery. Mainsoperated power supply fa this tube gives 8kv striking and 1.25kv at5mA running. Complete kit with case £15. As above for 12V battery.Also £15. Our ref 15P22.

ORGAN MASTER is a three octave musical keyboard. It is beautifullymade, has full size (piano size) keys, has gold plated contacts and iscanplete with ribbon cable and edge connector. Can be used with manycomputers, request Information sheet Brand new, only £15 plus £3postage. Our ref 15P15.

FULL RANGE OF COMPONENTS at very keen prices are available fromour assoaate company SCS COMPONENTS. You may already have theircatalogue, if not request one and we will send it FOC with your goods.

HIGH RESOLUTION MONITOR. 9in black and white, used Philips tubeM24/306W. Made up in a lacquered frame and has open sides. Made foruse with OPD computer but suitable for most others. Brand new. E16 OA£5 post. Our ref 16P1.12 VOLT BRUSHLESS FAN. Japanese made. The popular square shape(4.5n x 4.5n x 1,75in). The electronically run fans not only consume verylithe current but also they do not cause interference as the blush typemotors do. Ideal for coaling computers, etc., or fa a caravan. £8 each.Our ref 8P26.MIN MONO AMP on p.c.b. size 4in x 2in (app.)Fitted Volume control and a hole for a tonecontrol should you require it. The amplifierhas three transistors and we estimate thecutput to be 3W rms. More technical datawill be included with amp. Brand new,perfect condition, offered at the very lowprice ol £1.15 each, or 13 fa £12.00.

J&N BULL ELECTRICALDept. PE 250 PORTLAND ROAD, HOVE,

BRIGHTON, SUSSEX BN3 5QT.

IAIL ORDER TERMS: Cash, PO or cheque with order. Please add £2.50service charge. Monthly account orders accepted from schools and publiccompanies. Access and B/Cird orders accepted - minimum £5. Phone(0273) 734648 a 203500.Fax: (0273)20377.

POPULAR ITEMS - MANY NEW THIS MONTH

JOYSTICKS fa BBC, Atari, Dragon Commodore, etc. All £5.00 each. Allbrand new, state which required.

TELEPHONE TYPE KEYPAD. Really first dass rear mounting unit Whitelettering on black buttons. Has conducive rubber contacts with soft clickoperation. Circuit arranged in telephone type array. Requires 70mm by55mm cutout and has a 10 IDC connector Price £2.00. Ref. 2P251.SUB -MIN PUSH SWITCHES Not much bigger than a plastic transistorbut double pole PCB mounting. 3 for £1.00. Our ref BD688.AA CELLS Probably the most popular of the rechargeable NICAD types.4 for £4.00. Our ref. 4P44.20 WATT 4 OHM SPEAKER With built in tweeter. Really well made unitwhich has the power and the quality for hifi 6.5in dia. Price £5.00. Our ref;5P155 or 10 for £40.00 ref. 40P7.MINI RADIO MODULE Only an square with ferrite aerial and solid dia.tuner with own knob. It is superhet and operates from a PP3 battery andwould drive a crystal headphone. Price £1.00. Our ref. BD716.BULGIN MAINS PLUG AND SOCKET The old and faithful 3 pin withscrew terminals. The plug is panel mounted and the socket is cablemounted. 2 pairs for £1.00 or 4 plugs or 4 sockets for £1.00. Our ref.BD715, BD715P, or BD715S.MICROPHONE Low cost hand held dynamic microphone with on/offswitch in handle. Lead terminates in 1 3.5mm and 1.25mm plug. Only£1.00. Ref. BD711.

MOSFETS FOR POWER AMPLIFIERS AND HIGH CURRENT DEVICES140v 100watt pair made by Hitachi. Ref 25K413 and its complement2S1118. Only £4.00 a pair. Our Ref. 4P42.Also available in H pack Ref 2S199 and 2SK343 £4.00 a pair. Ref 4P51.TIME AND TEMPERATURE LCD MODULE A 12 hour dock a Celsiusand Fahrenheit thermometer a too hot alarm and a too odd alarm. Approx50x2Omm with 12.7mm digits. Requires IAA battery and a few switches.Canes with full data and diagram. Price £6.00. Our ref. 6P12.REMOTE TEMPERATURE PROBE FOR ABOVE. £3.00. Our ref. 3P60.A REAL AIR MOVER Circular axial Ian moves 205 cubic foot per minwhich is about twice as much as our standard 4.5in fans. Low noisemains operated 6.5in dia, brand new. Regular price over £30.00. Ourprice only £10.00. Our ref I OP71.600 WATT AIR OR UQUID MAINS HEATER Small coil heater made forheating air or liquids. Will not corrode, lasts for years. Coil size 3in x 2inmounted an a metal plate for easy fixing. 4in da. Price £3.00. Ref. 3P78a 4 for £10.0.0w ref. 1 OP76.EX -EQUIPMENT SWITCHED MODE POWER SUPPLIES Various makesand specs but generally +-5, +-12V ideal bench supply. Only £8.00. Ourref 8P36.ACORN DATA RECORDER Made for the Electron or BBC computer butsuitable fa others. Includes mains adaptor, leads and book. £12.00. Ref.12P15.

PTFE COATED SILVER PLATED CABLE 19 strands of .45mm copperMI carry up to 30A and is virtually indestructible. Available in red a black.Regular price is over £120 per reel. Our price only £20.00 for 100m red.Ref. 20P21 or I of each for £35.00. Ref 35P2. Makes absolutely superbspeaker cadetNEW RR SENSORS Infra red movement sensors wit switch up to 500wmains, UK made, 12 month manufactoas warranty, 15-20m range will a0-10min timer, adjustable wall bracket. Only £20.00. Ref 20P24.MITSUBISHI 331N DISC DRIVES Brand new drives, 1/2 height doublesided, double density warranted. Our price £60.00. Ref. 60P5.10 MEMORY PUSHBUTTON TELEPHONES These are customer returnsand sold as seen. They are complete and may need sight attention. Price£6.00 Ref 6P16 a 2 for £10.00 Ref t 0P77 BT approved.NON -MEMORY PUSHBUTTON TELEPHONES. Same condition asabove with redal £3.00. OUT ref. 3P79. BT approved.SPECTRUM PRINTER INTERFACE Add a centronics interface to yourSpectrum complete with printer cable for only £4.00. Our ref. 4P52.SPECTRUM SOUND BOX Add sound to your Spectrum with this device.Just plug in. Complete with speaker, vokume contra and Neely boxed. Asnip at only £4.00. Our ref. 4P53.BBC JOYSTICK INTERFACE Converts a BBC joystick port to an Ataritype port Price £2.00. Ow ref. 2P261.TELEPHONE EXTENSION LEAD 5rn phone extension lead with plug orone end, socket on the other. White. Price £3.00. Our ref. 3P70 or 10leads for only £19.001 Ref. 19P2.LCD DISPLAY 4.5in digits supplied with connection data £3.00. Ref.3P77 or 5 for £10. Ref. 10P78.CROSS OVER NETWORK 8 Ohm 3 way for tweeter midrange andwoofer nicely cased with connections marked. Only £2.00. Our ref. 2P255a 10 fa £15.00. Ref. 15P32.REVERSING LIGHT ALARM Fits to car reversing light and sounds whenreversing. Only £2.00. Our ref. 2P248.BASE STATION MICROPHONE Top quality unidirectlonal electretcondenser mic 600r impedence sensitivity 16-18KHz - 68db bull in chinecomplete with mic stand bracket £15.03. Ref. 15P28.MICROPHONE STAND Very heavy chromed mic stand, magnetic base4in high. 0.00 if ordered with above mic. Our ref. 3P80.SOLAR POWERED NICAD CHARGER 4 Nicad AA battery charger.Charges 4 batteries in 8 hours. Price £6.00. Our ref. 6P3.MAINS SOLDERING IRON Price £3.00. Our ref. 3P65.SOLDERING IRON STAND Price 13.00. Our ref. 3P66.SHARP PLOTTER PRINTER New 4 colour printer originally intended forSharp computers but may be adaptable fa other machines. Completewith pens, paper etc. Price £16.00. Our rel. 16P3.CAR IONIZER KIT Improve the as in your car, dears smoke and helpsprevent fatigue. Case req. Price £12.00. Ow ref. 12P8.NEW FM BUG KIT New design with PCB embedded coil 9v operation.Priced at £5.00. Our ref. 5P158.NEW PANEL METERS S0UA movement with three different scales thatare brought No view with a lever. Price only £3.00. Ref. 3P81.STROBE LIGHTS Fit a standard edison screw hght fitting 240V 40/min.dash rate available in yellow, blue, green and red. Complete with socket.Price £10 each. Ref. 10P80 (state colour required).EXTENSION CABLE WITH A DIFFERENCE It is flat on are side makingit easy to fix and look tidy. 4 core, suitable for alarms, phones etc. Ourprice only £5.00 for 50m reel. Ref. 5P153.METAL PROJECT BOX Ideal for battery charger, power supply etc.Sprayed grey size 8in x 4in x 4.5in. Louvred for ventilation. Price £3.00.Ref. 3P75.

A t long last the Hubble SpaceTelescope is in orbit. After apreliminary failure, it was launched

safely; there was a further hitch in unfurlingthe 'solar panels', built by British Aerospace,which provide the telescope's power, but allthe problems now seem to have been solved,and the outlook seems to be 'set fair'. Nodoubt we will have some major results in thenear future.

Meanwhile, Voyager I, which by-passedJupiter in 1979 and Saturn in 1980, has sentback the first 'snapshot' of the Solar System,showing the Sun and various planets. Like itstwin Voyager 2, it will never come back; it isleaving us permanently - and in manymillions of years hence it may still betravelling between the stars. We may hope tokeep track of it until about 2020, but then weare bound to lose it.

The Magellan probe is still en route forVenus, and will reach the neighbourhood ofthat hostile world in August. The Galileoprobe continues its tortuous route for Jupiter,though it will not arrive until 1995. Finally,efforts are at present (mid -May) being madeto re -activate the camera of Giotto, the space-craft which passed through the head ofHalley's Comet in 1986. If these efforts aresuccessful, then Giotto may be sent on torendezvous with a second comet.

THE MISSING LINK

Over the years many searches have beenmade for 'Brown Dwarfs' - objects which are

BY DR PATRICK MOORE CBE

The HST is safely inorbit and will

enhance our view ofthe universe, Brown

Dwarfs included.

not quite stars, but not quite planets. A normalstar, such as the Sun, begins its career bycondensing out of a cloud of gas and dustknown as a nebula; as it shrinks, under theinfluence of gravity, it heats up, and when thecentral temperature reaches a certain criticalvalue (around 10,000,000 degrees C.) nuclearreactions begin, so that the star starts to shine.

If the mass of the fledgling star if less than8% that of the Sun, the temperature will neverbecome high enough for nuclear reactions tobe triggered off, and the body will simplyglow dimly because of gravitationalshrinkage. This means that it will qualify as a`Brown Dwarf', with a surface temperature ofno more than 2000 degrees. After only abouta thousand million years, which is not long onthe cosmical scale, it will cease to glow at all.

Because they are so dim, Brown Dwarfsare difficult to find, and until recently it hadnot been definitely proved that they exist.Now, however, we may have the first definiteproof. At the Royal Observatory, Edinburgh,Dr. Mike Hawkins was examining platestaken with the UK Schmidt Telescope inAustralia when he found objects which wereextremely red, and looked very much asthough they could be genuinely dim.

The first step was to measure the distancesof these objects. One of them, knownprovisionally as MH 18, was examined andfound to be no more than 68 light-years away.This means that it was very dim indeed, witha luminosity no more than 1/20,000 that of theSun and with about 5% of the solar mass. Thismeans that it cannot be a 'tree' star, and putsit firmly in the Brown Dwarf range.

THE JUNE SKYnfortunately, Austin's Comet, which / discussed last month,has failed to come up to expectations. Instead of being abrilliant object in the dawn sky, with a brilliant head and a

long tail, it has been difficult to see at all without optical aid, andthough it will remain on view during June it will not becomebrighter now.

Comets, alas, are always unpredictable. According to currenttheory they are very ancient objects, and there is a whole cloud ofthem orbiting the Sun at a distance of around one light-year(approximately 6 million million miles). When a comet is perturbedfor any reason, it may 'fall in' toward the Sun, and when it enters theinner part of the Solar System we see it. Austin's Comet is probablypaying its first visit - and its last, as it has been affected by the pullsof the planets and thrown into an open path, so that it will nevercome back.

But though it was thought to contain a great deal of `dust', whichwould produce a long tail, this has not been the case, and we mustagree that as a spectacle it has been a great disappointment. Whenwe will next see a bright comet is uncertain; the last really 'great'comet was that of 1910, so that our own century has been verybarren of them!

During June, Mercury is technically an evening object, but willnot he seen with the naked eye from Britain. However, Venus isbrilliant before dawn, in the eastern sky, and cannot he missed,because it is so much brighter than any other star or planet (at itsbest, it can even cast shadows). Telescopically it is now 'gibbous' -that is to say, more than half hut less than full. No telescope canshow its actual surface, which is always concealed by its densecloudy atmosphere.

Mars, on the other hand, has a surface which is not veiled htcloud. It too is a morning object, and is now becoming bright,though its apparent diameter is still small. It will reach oppositionnext November. Of the other planets, Jupiter is still on view in thewest after dark, and the three outer giants (Saturn, Uranus andNeptune) are all in Sagittarius, the Archer, so that they rise wellbefore midnight. Saturn's rings are still wide open, and in anytelescope the planet is a beautiful sight.

Uranus, which is just visible with the naked eye, lies between thetwo stars Mu and Lambda Sagittarii. Telescopes show it as a pale,greenish disk. Neptune, at magnitude 7.7, is below naked -eyevisibility.

There are no eclipses this month, and no major meteor showers.The Moon is full on June 8, and new on the 22nd.

The starry sky during June evenings is dominated by the 'SummerTriangle' of Vega in Lyra, Deneb in Cygnus and Altair in Aquila.Look in particular for Cygnus, which is often nicknamed theNorthern Cross for obvious reasons; Deneb, 70,000 times asluminous as the Sun, is its brightest star. The Milky Way runsthrough Cygnus, and then through Aquila before running down tothe southern horizon in Sagittarius. Here we find the glorious star -clouds which hide our view of the centre of the Galaxy.

In the north-west Arcturus is prominent; later, the Square ofPegasus starts to come into view in the east. This is the best time ofthe year to see Antares in Scorpius (the Scorpion), the very bright,red star not far above the southern horizon. Scorpius is a splendidconstellation, but unfortunately it is well south of the celestialequator, and part of it remains permanently below the Britishhorizon.


If these observations are correct - and thereseems little doubt that they are - then we havethe first proof of a genuine Brown Dwarf. Nodoubt there are many others, and they mayprovide some of the 'missing mass' which weknow to exist in the Galaxy and which wecannot actually see.

What is the border between a BrownDwarf and a planet? It is difficult to say;Jupiter, for example, does not send out anylight of its own, and we regard it as a planet,while MI -I 18, perhaps ten times as massive asJupiter, is on the other side of the border -line.But in any case, Brown Dwarfs arefascinating objects, and they may have a greatdeal to tell us in the years to come.

Photos: The HST primary mirror, and thecomplete telescope ready for launch. Perkin -Eimer Corp, and Lockheed Missiles andSpace Co. Inc supplied the photos.

Astronomy NowBritain's leading astronomical magazine

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June issue on sale now - price El .50!Published by Intra Press - Publishers of Practical Electronics


TECHNICIAN GRADE 7Salary £14,445 - £16,467 (inc.)

To be responsible for providing a technical service in the Microprocessor Control andElectronics section of the Department of Engineering. Duties to include responsibility forensuring satisfactory maintenance, security and safety of areas and equipment within thesection, and the development, construction, testing and maintenance of apparatus forteaching purposes. Applicants require one of the following: HNC, HND, BTEC Higher,Advanced City & Guilds or an equivalent qualification in electronics. In addition 10 yearsrelevant experience (including training period) is required.

For further details and application form please contact Janice Garry atthe above address or telephone on 081-870 2241 Ext. 315.

Closing date 15th June 1990

Wandsworthan equal opportunity employerAll applicants are considered on the basis oftheir suitability for the job irrespective ofdisablement, race, sex or marital status.

ELECTRONICS

Are YOU looking to improveyour career in Professional Sales?We can offer an initial package of 15K+ pa with realpromotional prospects to Field ManagementDue to continual planned growth, MPS, which is a division ofMaplin Electronics, wish to recruit a

Sales and ApplicationsEngineer

Ideally residing in the Reading area, applicants should have asound knowledge of electronics and preferably be qualified toTec II or above. In addition, candidates should possessprevious sales experience plus the determination to succeed.We offer a negotiable salary package plus a company car andgenerous fringe benefits.

To apply, either telephone for an application formor sendyour CV to:

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BASIC TUTORIAL M*

e concluded last month withInvestigation 4, a mosfet switch.Let's look at a variation on that

theme (Figs 10 to 14 were shown lastmonth):

Investigation 5 - a mosfet touch switchYou need: the Schmitt trigger circuit of

Fig.10.The mosfet switch circuit of Fig.13, with

the 10cm wire connected to the gate (VR I notrequired).

The meter is not required in either circuit.Remove the led from the mosfet switch

and re -position R4 so that it connects thecollector to TR1 to the +6V line.

Remove VR1 from the Schmitt trigger.Run a short wire from the drain of the

mosfet to the 4k7 resistor (RI, Fig.10) toconnect the two circuits together.

Now try touching the wire with yourfingers and observe what happens to the led.You could replace the led and its resistor (R4,Fig.10) with some other load, such as afilament lamp (Module 2), a solid-statebuzzer (Module 5), or a relay (Module 7).

OTHER MOSFETS

The mosfet illustrated in Fig.12 and used inthe investigation is described as an n -channel

junction. The conductive part of the bar, then -type region, has been narrowed or depleted,so increasing the resistance of the bar andreducing the flow of current.

Enhancement and depletion transistorswith a p -channel are also manufactured. In thep -channel enhancement mosfet (Fig.16), thebar is of n -type silicon, with a region of p -type at each end. When the gate is negativelycharged, electrons are repelled, creating acontinuous p -type region from end to end. Inthe p -channel depletion mosfet (Fig.17) thebar is of p -type material. When the gate ispositiyely charged, electrons are attractedtoward it, creating a non -conducting n -typeregion. This restricts flow of charge carriers(holes) to a narrower region.

Vmos transistors are a special type ofmosfet in which the regions of the transistorare formed in deep V-shaped grooves cut in aslice of silicon. Both n -channel and p -channeltypes are available. The geometry of thetransistor allows a wide conducting region sothat large currents can flow withoutoverheating the transistor. This makes themsuitable for high -power applications. Somecan pass up to 30A, and others are rated at upto 125W. It is remarkable that such a largeamount of power can be controlled by theminute electric charge accumulated byrubbing a piece of plastic with a cloth.

+VE

Is= DRAIN

GATE

CURRENT Is

OF HOLESI=1SOURCE

OV

p -TYPE

SILICONn- TYPESILICON

IRONSEPELLED

p- CHANNELCREATED

p - TYPESILICON

Fig 16. P -channel mosfet with gatenegatively biased, and (Fig 17) withgate positively biased.

Id = Is

GATE

CURRENT

OF HOLES

SOURCE

Is

+VE

n- TYPEREGIONCREATED

p - TYPESILICON

BASIC ELECTRONICSenhancement mosfet. It is called 'n -channel'because the current flows through a channel ofn -type silicon. It is called 'enhancement'because the channel is created by increasing orenhancing the number of negative chargecarriers (electrons) in the channel.Investigation 4 showed that this is not an all -or -nothing effect. Conduction increasesgradually as the gate is made more positive.The mosfet behaves like a variable resistor.

We can also make a transistor that worksthe other way round (Fig.15). This is an n -channel depletion mosfet. The bar is madeentirely of n -type silicon, so it conducts freelywhen the gate is at zero voltage. If the gate ischarge negatively, electrons are repelled fromthe region close to the gate. This creates aregion of p -type material, through whichconduction can not occur owing to the pn

Fig 15. N -channel depletion mosfetwith gate negatively biased.

Id

DRAINId = Is

GATE

CURRENT

OF ELECTRONS I s

DJ G19251

+VE

p- TYPEREGIONCREATED

n- TYPESILICON

SOURCE

OV

By Owen Bishop

Part 7 - more ontransistors

How to score agood try on thetouch -line and

never be off -sidefor frost or fire.

DARLINGTON PAIR

Not to be outdone, the bipolar transistorscan also control a large current by the touchof a finger.

Investigation 6 - Darlington pair touchswitch

You need: battery box (6V)R1 330 ohmsLPI 6V, 0.06A filament lamp (Module 2)

(or use an led and 180 ohm resistor)TR I , TR2 ZTX300 npn transistor (2 off)Set up the circuit of Fig.19 as in Fig.20.

Investigate various ways of switching on thetransistors by supplying a very small currentto the base of TR I. Design a rain or 'tank full'alarm circuit. If you want to try switchinglarger currents, a power transistor such as a

Fig 18. Symbols for mosfets.

d d d d

n -CHANNEL p- CHANNEL

0.1 G 19281 ENHANCEMENT

n- CHANNEL p- CHANNEL

DEPLETION

PRACTICAL ELECTRONICS JULY 1990 -19

+6V

INPUT R1 330.11

OV

LP16V

0-06A

TR2ZTX300

1111G1929J

+6V

O

L01619301

LP1

Figs 19 and 20. Circuit and layout for Investigation 6.BD13 I can be used for TR2 (it may need aheat sink).

Explain the action of the two transistorsconnected as a Darlington pair.

MOSFETS REPLY

As hamburger is to cheeseburger, so

Darlington is to fetlington. Devicescontaining two mosfets connected in a waysimilar to the Darlington pair are marketedunder the obviously but nonsensically derivedname of fetlingtons. A very small change ingate potential produces a large change in thecurrent flowing through the transistor pair.

MODULES OF THEMONTH

8 Transistor switchThis has a similar circuit to Fig.7 last

month, but the protective diode is omittedbecause this is provided on the relay module.There is an additional transistor (TR2) whichturns on an indicator. The led is connectedbetween the collector of TR2 and the OV rail.This is a different arrangement to thatemployed in the other circuits described

earlier. When TRI is off, TR2 is on, and thevoltage across the led is too small to light it.When TRI is on, TR2 is off, and the voltage atits collector is enough to light the led. Theswitch supplies up to 100mA to the load. It issuitable for switching a lamp or a relay, but notthe usual type of small electric motor. (Fig.21).

Parts required: RI 680 ohms; R2 27k; R3180 ohms; DI TIL 209 or similar led; TRI,TR2 ZTX300 npn transistors (2 off); SKT1 3 -way pc socket; SKT2 2 -way pc socket;stripboard 63mm x 25mm (Vero 15354).

9 Schmitt trigger switchThis has the circuit shown in Fig.9, except

that the led and its resistor are omitted - Fig.22.Parts required: RI, R3 4k7 (2 off); R2 1k,

R4 100 ohms; TRI TR2 ZTX300 npntransistor (2 off); SKT 1 3 -way pc socket;SKT2 2 -way pc socket; stripboard 63mm x25mm (Vero 15354).

SYSTEM OF THEMONTH

Previously we promised to improve on thefire -alarm system. The solution to the problemof intermittent action is to use a Schmitttrigger. The Schmitt trigger module takes a

A

B

E

F

H

J

5 10 15 20 24

iftV (*), 0

0. A1TRZ

(0)

ws't

Fig 21. Module 8. Transistor switch.

Fig 22. Module 9. Schmitt trigger switch.

A

B

E

F

H

1 5 10 15 20 24.

r:

V

SdR1

aTo

KZ.

TC9''

I LOAD

1111619311

I LOAD

lamp, a buzzer of a relay as its load. Thedifficulty with this is that is works the wrongway round. As temperature increases above theupper threshold, the load is switched off. Thismakes a good frost alarm (Fig.23) but needsmodifying for use as a fire alarm. There arethree approaches:

(1) If the load is a relay which switches alamp or bell, wire the alarm device to contact1, so that the alarm is switched on when therelay coil is off

(2) Use a different heat sensor module, withthe thermistor and resistor swapped round.

(3) Use the transistor switch as an inverter(Fig.24). A resistor of say lk is connected as itsload. Output falls from almost 6V to about IVwhen the switch is on. This output is fed to theSchmitt trigger. This is another example of usinga resistor to 'convert a current into a voltage'.

The system of Fig.23 is also applicable as athermostat. The load of the trigger is a heater -a heating coil or a filament lamp. There isfeedback from the heater to the sensor. Thefeedback is negative in the sense that risingtemperature leads to the heater being turnedoff; falling temperature leads to the heaterbeing turned on. The result is a stabletemperature.

DISCUSSION

Investigation 5: The effects are the same asin Investigation 4, except that the switchingaction is more definite, due to the Schmitttrigger. Touching the wire puts the led firmlyon. Prolonged contact may cause the led tostay on after the finger has been removed.Eventually the excess charge leaks away intothe air and the led goes out.

Investigation 6: As soon as the basecurrent to TRI is enough to cause a collectorcurrent to flow, this collector current becomesthe base current of TR2 and causes an evenlarger collector current to flow. If, for eachtransistor, the collector current is 100 times thebase current, the collector current of TR2 is10000 times the base current of TR1.

TEMPERATURESENSOR

16)

ID.1619331

SCHMITTTRIGGER

(9)

OR

AUDIBLEWARNINGDEVICE

(5)

LAMP(2)

Fig 23. System of a frost alarm or thermostat.

Fig 24. System of a fire alarm.

TEMPERATURESENSOR

(6)

11:M61934.1

1-41.TRANSISTOR

SWITCH(8)

INVERTER

SCHMITTTRIGGER

(9)

AUDIBLEWARNINGDEVICE

(5)


READERS LETTERS L.

OBITUARY

We were saddened to hear of thedeath on March 5th 1990 of CyrilBogod. Cyril was the founder of theBritish Amateur Electronics Cluband its Chairman for 24 years untilhis death. Although I never metCyril, I had spoken occasionallywith him on the phone and wasaware of the devotion he gave to theBAEC and its members. I know thathe was held in great esteem by mypredecessors Fred Bennett andMike Kenward, and they bothsupported the club. In the latestissue of the club's newsletter is aquote from Fred (PE's foundingeditor) in appreciation of Cyril'sefforts:

"In this materialist world wherethe 'every man for himself' idea isso prevalent, altruistic works shineout as brilliant beacons of hope,confirming that the spirit ofgenerosity and mutual help is stillvery much a part of man's natureand will not be vanquished."

John Margetts, Secretary of theBAEC since its inception, commentsthat Cyril "was a kind, gentle manwith a quietly determined manner,and it is due to his enthusiasm andgenerosity that the BAEC has beenso successful and able to withstandthe rigours of time. He was alwaysready to help beginners inelectronics, particularly the young,and he achieved a great deal ofsuccess in this aim through theregular weekly meetings of the clubat Penarth, where many novicesgraduated into knowledgeableamateurs. His passing also signifiesthe end of an era in the history ofthe BAEC."

Cyril will be sadly missed bymany people and we send ourcommiserations to his family and tothe BAEC.

Herbert Howard has assumedresponsibility for the BAEC and weknow that the club is in good hands.Many of you will recall that 1publicised the BAEC's call for newmembers in PE May 90. Afterpublication of that issue, I receivedthe following kind letter fromHerbert.:

ONWARDS BAEC

Dear John,I am writing to thank you for your

enthusiastic advocacy of the BAECon page four of PE May 90. I am surethat other members of the committeewould join me in expressing ourgratitude. Before your columnappeared the club had already gainedseveral new members as a result ofadvertisements in PE last August andagain in January this year. Since yourcomments we have had several morejoining. I know that Cyril Bogodenjoyed your support over the years,and I am glad to know that I can counton continuing support from you.

TRACKFEEDBACK

Like Cyril, I am an electronicshobbyist and have never beenprofessionally engaged in radio orelectronics (unless you count six yearsin the Signals Branch of the R.A.F.during the last war, when I worked onradar). Nor have I any journalisticexperience; after taking a degree inBiology I taught the subject atsecondary level till I retired in 1975.My interest in "wireless" started atabout the time the BBC was born; Ibecame an avid reader of WirelessWorld and other magazines and wassoon into building 'hi-fi' amplifiers. Iran a school radio club and wasresponsible for all the audio-visualequipment (including constructingmuch of the electronics side), andwhen I retired I continued at theschool as part time audio-visualadviser. Last year I gave that job up,so I am now able to give a good dealof my time to the affairs of the BAEC.

During the past few weeks I havespent a lot of time at Cyril's house,going through the mass of papersrelating to the club, and trying to sortout all the equipment and componentswhich Cyril had accumulated. He leftall his electronic equipment and booksto the club, so the committee has thejob of deciding what to do with it, andthat is by no means easy; it seems asthough Cyril never threw anythingaway. I get the impression thatwhenever Cyril saw something thattook his fancy he bought it, and veryoften never got round to doinganything with it. If he needed an is orother component he bought a dozen(or a hundred)! I shall try to get someof the useful stuff into the hands ofthe several young beginners who haverecently joined the club.

I noted in looking through theMay issue of PE that you are makinga special offer of a reducedsubscription rate to teachers andstudents, and I wondered if you couldconsider extending this to includemembers of the BAEC.

Incidentally, I have been readingPE since it first came out.

Herbert Howard, B.A.E.C., 41Thingwall Park, Fishponds, Bristol,

BS16 2AJ.

Yes, certainly, we shall be verypleased to extend the specialteacher/ student subscription rateoffer to BAEC members. The normal12 month's subscription rate for UKreaders is reduced by 10% forteachers and students. BAECmembers may take advantage of thisspecial rate by enclosing proof of

their BAEC membership status withtheir payment, writing to theSubscription Department at theeditorial address.

Readers interested in joining theBAEC, a very worthwhile club,should write to Herbert at the addressat the end of his letter.

Our Publisher, Angelo, joins me insending our best wishes to Herbertand the BAEC. Ed

HOBBY HAWK

Dear John,I thought that, in your "Hobby

Horse" Editorial of PE April 90, youreffusive defence of electronics as ahobby was rather unfair on theanonymous critic. After all, he/sheshould have had the right to reply! Soin view of their absence I havedecided to take up the cudgel on theirbehalf.

To do their critique justice itwould be necessary to review thewhole scene and no-one can deny thatthings in the electronic -hobby world

. ain't quite what they used to be. Itseems to me that the whole of thepioneering/creative spirit has droppedout of the business. It's not your faultthat you do not appreciate this; howcould you, when you are too young tohave experienced the thrill of being'in' on the discovery of the age. As asecond generation in practicalelectronics you have inherited a faitaccompli, the honours have all beenwon, the caravan has passed by. Atmost all you can achieve is a steadytrot with a flagging horse. However,we live in a changing world and whoknows what fate has in store.

I am enjoying taking PE againafter a lapse of 16 years. I have a fullset of issues from 1964 to 1974 (theformative years) and made a fewsmall contributions to the articlesduring those years.

George Dunning, Hassocks,Sussex.

The head of the caravan hasindeed passed by, but not its bodywhich is still well attended bydevotees and those who areencouraged to join along the route.There are many honours still to bewon, many horizons still to becrossed. The world of electronics hasyet many adventures to be exploredand part of my job is to encouragepeople to participate in them.Although commercial designers andmanufacturers are the pathfinders, therank and file amateur hobbyists still

have much to gain from electronics,and much to offer in areas unsuited tocommercial exploitation.

Having been involved inelectronics, in various ways, foraround three decades, I for one havefound in myself an increasingaddiction to electronic creativity, andrefuse to believe that diy hobbies ofthis type are anachronisms.Comments from anonymous critics,such as those 1 highlighted in April,increase my determination to fight forwhat 1 believe in. Ed.

ARMAGEDDON

Dear Mr Becker,I am writing to you once again

with regard to my previous letters, andnow that all of my work is completed,I do want to let you know the finalresults of my work.

Because of my work on thesolution to the origins of the universe,I wish to tell you that the final battlefor good against evil will be fought onthe Earth, and at the fusion site atCulham in Oxfordshire, when theyattain fusion of the plasma in thefusion chamber it shall release thepower of Armageddon on the Earth,and the Apocalypse will havehappened.

It will not destroy all the world, butit will serve to let mankind know thatGod exists, and that we are all part ofhis creation, and it will be the start of anew world, where we can go to hispromised land if we repent of our sins.

As far as I am concerned the aboveinformation is correct, and please dokeep this letter on file for futurereference.

The consequences of me givingyou this information, do not concernme at the moment, and as far as I amconcerned I can await the results of thefusion project experiments patiently,because I do know that is all I can do.It does not concern me if I am believedor not at the moment, because I doknow that I will be proved correct bythe results of the events at Culham inOxfordshire where the "JET"experiments are being conducted at themoment.

Armageddon is fully explained inthe book of Revelation 16.

R.D.Caldwell, Warrington,Cheshire.

The United Kingdom AtomicEnergy Authority (UKAEA) areresponsible for the activities atCulham. Their spokesman at Harwelladvised me that hot fusion plasma hasalready been achieved, lasting for 25seconds at 8 to 10 million degreesCelcius. It is expected that a prototypefusion reactor will come into operationin the year 2000.

However, the spokesman pointedout that Nostradamus predicts that theworld will end in 1996. It has to headmitted, though, that Nostradamus(1503-1566) did not have the benefitof modern computational technologyto aid him with his prophecies. Ed


PE COMPETITION RESULTSWINNING BONANZAS!

Ten lucky winners in ourApril 90 competition

have won a bumper bundlebonanza:

Over 550 pages of invaluable information andcircuits for robotic and automation control. Underfull positive Post Office feedback control we'vedespatched the two prize books, "The RobotBuilder's Bonanza" and the "Remote ControlHandbook", to the following fortunate readers:

A.J. Fletcher, Gilwern, Gwent.J.C. Sutton, Misson, Doncaster, S.Yorks.

Roger Ingle, Great Leighs, Chelmsford, Essex.Gerard Butler, Dun Ladghaire, Co Dublin, Ireland.

R.S. Orr, Hertford, Herts.Gordon Rainbird, Ongar, Essex.

John Findlay, Bournemouth, Hants.N. Thomas, Lyneham, Wilts.

Michael Drake, Callander, Perthshire.Liz and Crystal Hanson, Bracknell, Berks,

(both names on the same form - is this what isknown as double -entry book-keeping?).

Congratulations to all of you,and our grateful thanks to John Wiley

and Sons Ltd, and Bernard Babani(Publishing) Ltd, respectively, for kindly

supplying the books.

THE ANSWERS

We asked you who is reputed to have first used the word Robot. It was,of course, Karel Capek who coined the word to describe a person ofmachine -like efficiency for the 1920 play "R.U.R." ("Rossum'sUniversal Robots"). Only six of you didn't know the correct answer,all believing that the honours go to Arthur C. Clarke. I save DouglasAdams as an option in the question, but I guess you're all too familiarwhich his fame for "Hitch -Hiker's Guide to the Galaxy" to fall forthat! Frank Herbert, the fourth option given, is one of my favourite SFwriters, probably best known for his "Dune" series.

Isaac Asimov defined the First Law of Robotics to be: "A robot maynot injure a human being, or, through inaction, allow a human being tocome to harm". Every single one of you got that right! Of the optionalnames given, I hold in esteem Plato for being a Greek philosopher,Faraday for his research into electrical charge (the unit of capacitanceis named after him), and Napier for his work in mathematics. TheMinistry of Defence has numerous claims to fame!

THE

Remote Control

ILDER'S

Handbook

ROBOT BU

99 INEXP1

ROBOTICSR1

OWE !I SISHOy

GC

The third question produced answers that were virtually equally infavour of each of the names offered as to who was the odd one outamongst Marvin, Hal, R.Daneel, R2D2, Joe 90, and Dalek. When I setthe question I had Hal in mind as the exception since I felt that theothers, in a broad sense, could all be regarded as robots/automata. Halwas simply the name given to the sophisticated talking computer inArthur C. Clarke's book/film "2001". Marvin was the depressed robotin the "Hitch -Hiker's Guide" series. R.Daneel was a robot created byIsaac Asimov, and which, as a logical detective, solved a particularlybaffling murder in the book "Caves of Steel" (I believe), a sort ofcybernetic Hercule Poirot! The film "Star Wars" brought fame forthe robot R2D2, though I can't remember whether he was the short,fat, squeaky one, or the tall, erudite, silver one!

Perhaps, strictly speaking, Joe 90 was not an automaton, for in realityhe was a puppet, used by Gerry and Silvia Anderson in their famous tvseries of the same name. (I worked for the Andersons on their earlier"Four -Feather Falls" puppet series, but left before the making of"Joe 90". Even then, about 1959 or so, they had the puppets' mouthmovements electronically synchronised to the recorded speech tracksignals.)

I'm not sure how Dr Who would define a Dalek, but to me itepitomises the concept of a malignant robot, and so I included it in thelist. Since you were all pretty equally split over the answers to thisquestion, I allowed all your answers to be equally valid and didn't takethem into account for the final draw!

I was not so lenient on Q4. Although Marvin is strongly associatedwith hitch -hiking and the Answer to Life (42, as some of you said), tome the endearing attribute for which I shall remember him is his acutedepression! (So, too, would you be depressed if you'd been stuck in acar park for most of eternity - a fate nearly worse than being stuck inLondon's traffic jams.) Most of you agreed with me, and obviouslysympathised with poor old Marvin.

No-one who had fully read their April 90 copy of PE should have gotQ5 wrong. The answer was given in Alan Pickard's text for the RobotCar! He named the axes of robotic arm movement as pitch, roll andyaw. I'm pleased to note that most of you had, apparently, read Alan'smoving words of wisdom.


ENTERTAINING ED!

I also suggested that you should write a limerick on the subject ofrobots. It was not a condition of the competition that you should, butwas simply a means by which you could entertain me. And some ofyou did just that, though not as many as I had thought might do so.Perhaps electronic creativity and worse -verse creativity are notnecessarily compatible attributes! But I'm sure you'll all be entertainedby the odd -odes that were sent in. Here they all are:

A robot was riding at randomWith a soldering iron on a tandem.

It fixed his dry jointsAnd other weak points

While he consumed blackcurrent and rum.

Liz Hanson of Bracknell, Berks, submitted that one. Need I point outthe current pun? I like the rhymes of random, tandem, and -rum. Iwonder what Liz consumed to come up this ditty?!

Paul J. Smith of Walworth, London SE17 obviously has a sporting eyefor final indignities:

There was a young robot from Leicester,Who got a job as a wrestler,

Three falls and a submission,Completely ruined his transmission,So now he works as a cash register.

Is Howard Barnes of Lower Shelton, Bedford suggesting that marriageand intelligence are inharmonious? :

We built a robot, all singing and dancing,And then put AI in the thing,

It made a mistake, once,Got its degree, not by chance,

So we gave it a wife, how confusing.

Malcolm Ives of Horncastle (I know it well), Lincs,might be said to take a pinker view of inter -species(!) relationships:

There was smart robot called FloydWhose life was almost a void

Til one day he metA girl called Collette

And now he's fathered an android.

Malcolm also submitted the next one - I'll leavecomments about the last line to you!

There was a mechanical manControlled by some rom and some ram

But when exposed to some raysHe went into a daze

And now he's a Kylie Minogue fan.

Eric A. Cook of Llanfrechfa, Gwent, is no stranger to me. The winnerof a satellite tv receiver in one of our competitions a couple of yearsago, he is a regular entrant to all our competitions. He titled hislimerick "Aye, Aye, Robert" (I wonder if he is making a punnedreference to Asimov's book 'I, Robot'?):

There was a young man called HicksWho said "There's now't 'Bob' can't fix!".

When given repair jobsBy his boss's hob -nobs

He just passed them to his robotics!

Eric comments that robotics should be pronounced here as Robert -Hicks!

If L.G. Wright of Bournemouth, Hants, is punning on the word 'Ed' hecould find it's his days that are numbered, not those of his tabloidrobot!:

A cynical robot called EdWas mistakenly digital fed,

Now asked for the keyTo a formula he

Recites figure tables instead.

It's probably just as well for Gordon Rainbird of Ongar, Essex, that wedon't publish full addresses:

There once was a robot called MossWho was rather fond of a doss

He'd sit in the sunHaving nothing but fun;

Some might called him my boss.

Ivan Roulson of Honiton, Devon shows signs of having already donesome robot building:

There once was a robot called BobbyWho was part of a constructor's hobby.

His wires were gold -But his tyres were bald

And his axes were all very sloppy.

If I were offering prizes for the limerick entries, which I'm not, Iwould probably have chosen Michael Drake of Callander, Perthshire,as the winner, providing he indemnified us against libel suits!

A certain inventor survivesDespite his companies' soars and dives.

He's known for computersAnd electric commuters

But he's really an android called Clive.

Michael Drake, determined not to duck theissue, sent in these two sketches with hislimerick. I suspect I recognise the caricatureas being of one of my favourite inventors.

Michael added a comment to the robot sketch, saying, "A limerick onrobots (plural); is valid since this 'robot' is a system of robotscombined to form a cooperative unity of functional flexibility"!

Revelling in the joys of imagination, Michael also sent in a hex codelisting, commenting:

"As requested, 'a limerick on robots' (Hexadecimal, Sinclair Spectrum1983): 22 41 20 6C 69 6D 65 72 69 63 6B 20 6F 6E 20 72 6F 62 6F 7473 22."

I don't know Spectrum Hex but I've worked out what it says, can you?(No prizes!) Well done Michael, thanks for the entertainment.

Thanks, too, to all who sent the other limericks. We'll be havinganother competition for you soon, so don't forget :

PE Servos You Right!


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PracticalElectronics

Classified Works!

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Dear Mr. Bonner,

Just a line to say that I was

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Please see our displayadvertisement in this issue

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We stock a large range of electronic components,test equipment telephone accessories computer

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Resistors 1/4 W 5% carbon (E12) Ip metal film 1% .._ __-..____3pRes Pack 85 different E12 values + zero ohm link total content1000 resistors. --_-_18.95

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Software Applicationsand Dev?topment

The latest issue available from allgood newsagents or direct from:

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ESTABLISHED 15 YEARSTel: 0945 584188 - Fax: 0945 588844

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Gould PSU 0-30 @5A Limited gualtity only at 645.00 eachDual Data lead (BBC Micro bp 2 Disk Drives) £ 4.00 eachPower lead BBC( Micro b Disk Drive) £2.00 eachDual power lead (BBC Mao to 2 DEC Dives) £ 4.00 each68030 CPUs (The first orders get 10MHz chips) £3.50 eachZ80A CPU, CTC, P10 £120 each; DMA £2.00 £4.50 al 474LS TTL , pick and mix, 10a mae fa 10.12 eachTypes avaiable: '00 '02 "08 11 '13 15' '20 '21 '26 '27 '30 '3233' '38 '42 '75 '83 '96 '107 109 '122 132 '136 139 '145 151'153 157 '158 '163 '164 '166 '191 '193 '298 353"365 '6702764 EPROMS( Ex-eguipnent) 61.00 each27128 EPROMS £250 each256K Byte DRAM Modules, removed ken equipment £6.00

6116 2K Byte SRAM 61.10 each6264 8K Byte SRAM -15 £3.00 each -12 £3.80 each65256 32K Byte rams 67.00 each8K Byte NV ram chips 63.00 each 610.001u

20 pin al law mile IC sockets £0.50 (tai) - 6 4.00 (10040 pin di low profile IC sockets £0.60 (sill- 6 5.00 100Keyboard, 100 keys on board LCD & miao iA £8.00 each

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Smoke detectors, smal size £7.00 eachSmoke detectors, lace will escape light and pause feature £13.00 each

Prices induce postage. Add to Was below 65.00.All items new unless stated . Add 15% VAT to all prices.

Send an SAE fa our latest 1st a for more info.Dept PE, 314 Milton Road, Cambridge, C84 15.I Tel:

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Wide range of items available. Send for lists.

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EPROM Emulator £99Cross Assemblers £49Powerful Simulators £99EPROM Programmer £129

SMART CommunicationsTelephone 081-441 3890

N.R. BARDWELL LTD200 -Signal diodes 154148 £1 .00100 -Rectifier diodes 1N4001 £1.03100 -Zenner dlocles6V8 400mW £1.0030 -Assorted Da sockets up to 40 pin £1.0030 -Assorted SocketsCons. DI, Edge, I.D.C., Header etc £1.0030 -Transistors BC478 £1.0020-1Anature SP/CO Slide reaches £1.0020 -Magnetic ear pips + load and Plug £1.0075-Bectrolytics 4.7ul 63V £1.001-10 Watt stereo amplest, - 4 controls and data £2.95

Noes Include VAT, postage 6.0p. All items new.

Many other lines In stook.Shop open MorVSat 9.30-5.30. Lists S.A.E.

288 ABBEYDALE ROAD, SHEFFIELD S7 1FLTEL: (0742) 552888 FAX: (0742) 500689

30 DAYS GUARANTEE.PLEASE ADD VAT.

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MARCONI 2600S MILLIVOLTMETER AI VJAI 3000 F00 L 25MARCONI 144H 1.010-12 T0 72 MK HIGH QUALITY RF GENERATOR FROM

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E.G. CURSOR BAUM . PROTOCOL . BRILLIANCE ETC. HARDLY USED1.160

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* Series X Mixer Kite* up to 1.000 inputs* 60-1.100 mm faders, pots,panels and audio switches* 6-9 echo sends* versions for recording PA,radio, disco* circuit diagrams* From £9.92

Send 40p for catalogue to: K. Tek, c.O. Box 172a,Burbiton, !Surrey KT8 EITEN. Te1:01-399-3090

7,80 MICROPROCESSOR PROJECT BOARD

Bring your ideas to life with this versatile pcb. 10 by 8 inches, tapquality, through hole plated, double sided, full solder resists dreamto work with after veto. Support: Z80, 4MFI: clock, 8/161 EPROM,8kSRAM, up to 612k DRAM, two RS232 & two Centronics porta, keypad,Eon LCDs, battery back-up, PSU/-Z for bare pcb & manual by return of poet, 10 day money backguarantee. Also kite of components, software & project ideas. SAE forfurther details.

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PRACTICAL ELECTRONICS JUNE 1990 55

CLASSIFIEDS

COMPONENTS

SURPLUS/REDUNDANT ELECTRONICS

COMPONENTS WANTEDics- Tuners - Transistors - Valves - Diodes etc - any quantity

considered -immediate payment.

ADM ELECTRONICS SUPPLIES

Tel 0827 873311 Fax: 0827 874835

C.S. COMPONENTS (Est. 4 years). Suppliers &stockist of electronic components, electronicvalves, military equipment and spares. Obsoleteand hard to find products are our speciality. C.S.Components, Southfield House, 11 LiverpoolGardens, Worthing, West Sussex BN11 1R

SURVEILLANCE

Surveillance devices, lasers, Teslacoils, scramblers, ultrasonic and manymore, over 150 designs. Send SAE to:Plancentre, Old Wharf, Dynock Road,Ledbury HR8 2HSMICROTRANSMITTERS, VHF, Phonetransmitters Bleepers, Vox, Phonerecorders, kits or built. Also plans forhandheld EHT units, sonic guns etc.SAE to: P.O. Box 55 Cannock, Staffsfor free list.NEW VHF MICROTRANSMITTER KIT,tuneable 80-115 MHz, 500 metrerange, sensitive electret microphone,high quality PCB. SPECIAL OFFERcomplete kit ONLY £5, assembled andready to use £8.95 post free. Accessorders telephone 021 411 1821.Cheques/ P.O.'s to: QuantekElectronics Ltd, (Dept P.E.), 45aStation Road, Northfield,Birmingham, B31 3TE

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SYSTEMS SPECIALISTS

PCBS

We will make drilled PCBs to your

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Voice/Sound activated switches easyto follow diagrams and uses, only £1.00.Components and PCBs available fromHerrington, 63 Home Farm Road,Hanwell, London W7 1 NLBP 34 wanted. Practical Repair andRenovation of Colour TV's. Good pricepaid for book or photocopy. Contact: B.S.Smart 27 Knowle Road, Maidstone, Kent Tel:01-945-5338 (day) or 0622 51501 (eve. )Oscilloscope Xmod dual beam £60.00.R209 xmod reciever 1-20MHz £60.00. A41TXRX complete outfit £60.00. All withmanuals and leads. Larkspur morse keys newboxed £12.00. All prices include P&Pcollectors paradise. W -ton 20315.Pressure transducers stainless steel. 1.6 barguage £49, 4/6/25 bar absolute £39. Surplusstock at less than 30% cost. D.T. systems0992 550078.Message display hardware. Movingcharacter kits large displays, all can becontrolled from your computerport, detailss.a.e. Caddis Systems Ltd. P.O. Box 859,Basildon, Essex SS15 SJE.Swap Shop. Alarm controlpanel swap for 3Corgi trucks/2 trams (stock 8). 2 zone swapHornby Passenger steam type loco (56).Parcel insurance add £1 stamp. For callers,bells, strobes, sonics. Hockley, 61 DisraeliTerrace, Leeds LS11 6NP. (778444 Private).

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56 PRACTICAL ELECTRONICS JUNE 1990

S ome time this year the UK's cellularmobile radiotelephone service willhave taken on its millionth customer.

Relative to the country's total population ofabout 57 million this may not seem a lot. Butcompared with some other Europeanindustrialised countries it's a considerablesuccess for the service. For example, the UKmarket penetration is about five times thatachieved in France and Germany. TheScandinavian countries have done better, withapproximately 4% market penetration relativeto the UK's 1.8%, but they started about fiveyears earlier, in 1981.

BRITISH SUCCESS

All in all the British service is seen as agreat commercial success and as a model ofhow a public cellular system could be run.The formula for this success has been intensecompetition functioning under strictGovernment regulation. Right from the start

PRINCELY PRICING

To most people the cellphone is a somewhatexotic appliance. It's associated with yuppies,show-offs and generally very well-heeledtypes. You certainly need a good income to beable to afford one. The equipment costs about£250 for a carphone or £500 for a hand -portable, though you can also get these on leaseor short-term hire. Connection charge is about£60. A yearly subscription, or standing charge,is in the range £300 to £350. Finally, there isthe price of the calls themselves - 25p perminute or, in the London area, 33p per minute.An average bill is perhaps in the region of£1,000 per annum.

So, despite the price competitionmechanism, the mobile cellphone service is notreally for the ordinary private citizen. Most ofthe customers are in fact business users. Herethe cost can be justified by the commercialbenefit to the user of being able to keep intouch while on the move. For example,

THE MEGACELLPHONEthe Government decided that the schemeshould be run by two operating companieswhich would compete with each other forcustom. As almost everyone knows, these areCellnet and Vodafone. The Governmentlicensed each to provide and operate its owncellular network and associated services as aseparate business.

Cellnet is the trade name of TelecomSecuricor Cellular Radio Ltd, a jointsubsidiary of British Telecom (60%) andSecuricor (40%). Vodafone is the trade nameof Racal Vodafone Ltd, a wholly ownedsubsidiary of Racal Telecom.

The idea of the Government regulation isto make sure that the competition reallyworks to the benefit of the public - notably inproviding good service and keeping downprices. There are in fact two Governmentbodies involved in this. The DTI isresponsible for the overall technical andcommercial organisation of the service -including, for example, the frequencyassignments in the 900MHz mobile radioband and the two operators' licences. Thenthe Office of Telecommunications (OFTEL)acts as a 'watchdog' to look after the interestsof the public. It does this, for example, byenforcing the rules of competition,investigating complaints and making sure thatthe two network operators stick to theirlicences.

COMPETING CUSTOM

But although Cellnet and Vodafone arecompeting with each other for custom, thecompetition in the whole scheme is notrestricted to just this system level. If it were,there would be a danger that these twooperators could form a secret cartel and fixprices, to the detriment of the customer.

By Tom Ivall

The great success of theBritish cellphone has

been aided by sensiblelegislative controls, butis now hampered by lack

of air -space.In fact the competition extends outwards to

other companies. This is the result of a cleverand significant regulatory rule. A clause in thenetwork operators' licences expresslyprohibits them from selling cellphoneequipment or offering air -time service directlyto the customer. Instead this selling to thepublic has to be done through retailers calledservice providers. Some of them sell directlywhile others work through a network ofdealers.

So competition is spread out among alarger number of companies than just the twonetwork operators. This intensifies the contestfor market share and the custom of the public.Market forces therefore tend to bring downprices and improve the range of choiceoffered to the customer.

And this is not the end. There is anotherlevel of competition - between the suppliersof the actual cellphone equipment. A dozen ormore electronic equipment firms sell theappropriate products through a variety ofretail outlets like phone shops, hi-fi dealersand High Street multiple stores. The greatpity, though, is that the UK's electronicsmanufacturing industry is not benefiting here.Most of the manufacturers are foreigncompanies. Probably the biggest supplyingthe UK service are Motorola (USA) and NEC(Japan).

company representatives are reported to havetaken large orders over their cellphones whiletravelling. A freelance actor I know is able tosnap up work when away from the hometelephone. Other itinerant users include vets,surveyors, servicing and repair people andmembers of small businesses.

INCREASINGCONSTRAINTS

Even though price competition is always atwork the present system is likely to remain aminority service. The network operators don'texpect to be able to cope with much more thanabout two million subscribers in all. They areup against the physical constraints ofgeographical space, time and frequencyspectrum.

In practical terms these constraints show up,for example, in the increasing difficulty ofgetting new sites for base stations around thecountry and in the channel congestion nowcausing a certain amount of difficulty inmaking calls through the system. According torecent OFTEL figures, customers could notconnect into the network for 4.7% of the timeon Vodafone and 7.3% of the time on Cellnet.Peak period congestion is higher.

The future for public mobile radiotelephoneservices will depend very much on how the newdigital technology works out, operationally andcommercially, against the established analogueengineering. Telepoint is already with us -though this has the disadvantage that you can'treceive calls on your hand -portable. A pan-European digital system is being planned, whilein the UK the Government has now issuedlicences for companies to develop and operatewhat are being called personal communicationnetworks. Cost and convenience will determinethe final outcome.


PRACTICAL ELECTRONICS

BOOK SERVICEHere is your Editor's choice of books he

thinks will be of interest toelectronics and

computer enthusiasts

BEGINNERS AND EARLY STARTERS

Mini -Matrix Board Projects.R.A.Penfold. 112 pages. £2.50. OrderCode BP99Shows a selection of 20 useful and interesting circuits thatcan be built on a mini -matrix board of 24 holes by 10 copper

strips in size - an ideal book for early experimenters.

From Atoms to Amperes.F.A.Wilson. 160 pages. £3.50.Order Code BP254.For the absolute beginner. clearly explaining thefundamentals behind the whole subject of electricity andelectronics.

Electronic Projects for Beginners.F.G.Rayer. 128 pages. £1.95.Order Code BP48Specially for the newcomer to electronics who Is looking for a

book containing a wide range of easily made projects. Somecircuits need no soldering and many others show actualcomponent and wiring layouts.

Electronics Build and LearnR.A.Penfold. 128 Pages. £5,95. OrderCode PC 101Combining theory and practice. the book describes a circuit

demonstrator unit that is used in subsequent chapters tointroduce common electronic components and circuitconcepts, complete with practical experiments.

Practical Electronic Building BlocksR.A.Penfold. There are two books -Book 1 : 128 pages. £1.95.Order Code BP117Book 2 : 112 pages. £1.95.Order Code BP118Book t is about oscillators and gives circuits for a widerange. including sine, triangle. square, sawtooth and pulsewaveforms and numerous others from voltage controlled tocustomised is types.Book 2 looks at amplifiers, ranging from low level discreteand opamp types to is power amps. A selection of mixers.filters and regulators is included.

30 Solderless Breadboard ProjectsR.A.Penfold. Two books each of 160pages. Book 1 : £2.95. Order CodeBP107. Book 2 : £2.25. Order CodeBP113.Each project is designed for building on a Veroblocbreadboard and is accompanied by a description, circuit andlayout diagrams and relevant constructional notes. Many ofthe components are common to several projects. Book 1covers linear devices, and Book 2 covers cmos logic chips.

Beginners Guide to BuildingElectronic Projects R.A.Penfold. 112pages. £1.95. Order Code BP 227Shows the complete beginner how to tackle the practical sideof electronics and includes simple constructional projects.

SATELLITE TV

Satellite TV Installation Guide - 2nd edition John Breeds. £11.95. OrderCode STV1Full of vital information for any competent diyer who wishes to install a satellite tv antenna and obtain optimumreception quality.

An Introduction to Satellite TelevisionF.A.Wilson. 112 pages. £5.95. Order Code BP195Informative answers to many of the questions about this communications revolution. The information is presentedon two levels, one aimed at the complete beginner, the other at professional engineers and serious amateurenthusiasts.

TEST AND MEASUREMENT

Getting the Most from YourMultimeterR.A.Penfold. 112 pages. £2.95. OrderCode BP239There's more to what you can do with a meter than meets thecasual eye. The book covers the basics of what you can dowith analogue and digital meters and discusses componentand circuit testing.

Test Equipment ConstructionR.A. Penfold £2.95.Order Code BP248Describes in detail how to construct some simple andinexpensive, but extremely useful, pieces of test equipment.

Oscilloscopes 2nd EditionI.Hickman. £12.95. Order Code NT3Subtitled 'How to Use Them, How They Work the book Isillustrated with diagrams and photographs and Is essentialreading for any one who wants to know about scopes, fromfirst principles to practical applications.

How to Get Your Electronic ProjectsWorking.R.A.Penfold. 96 pages. £2.50.Order Code BP110.Essential reading for anyone who wants first-time success inproject assembly. Covets tracing mechanical faults as well astesting for failures of active and passive components of mosttypes.

AUDIO AND MUSIC

Introducing Digital AudioI.Sinclair. 112 pages. £5.95.Order Code PC102A non -mathematical introduction to the new digitaltechnology, discussing the principles and methods involved indevices such as cd. dat and sampling.

Electronic Music ProjectsR.A.Penfold. 112 pages. £2.50. OrderCode BP7424 practical constructional projects covering fuzz, wah.sustain, reverb, phasing, tremolo etc. The text is split intofour sections covering guitar, general, sound generation andaccessory projects.

More Advanced Electronic MusicProjectsR.A.Penfold. 96 pages. £2.95.Order Code BP174Complementing BP74 by covering more advanced andcomplex projects including flanging, chorus, ring modulation.plus a selection of drum. cymbal and gong circuits.

Computer Music ProjectsR.A.Penfold. 112 pages. £2.95. OrderCode BP173Shows how home computers can produce electronic musicand covers sequencing, analogue and Midi interfacing. digitaldelay lines and sound generators.

Practical Midi HandbookR.A.Penfold. 160 pages. £5.95.Order Code PC103A practical how -to -do -it book for musicians and enthusiastswho want to exploit the capabilities of Midi. Coverskeyboards. drums. sequencers. effects. mixers, guitars. andcomputer music software.

Midi ProjectsR.A.Penfold. 112 pages. £2.95. OrderCode BP182Practical details of interfacing many popular home computerswith Midi systems. and also covering Midi interfacing toanalogue and percussion synths.

Electronic Synthesiser Construction.R.A.Penfold. 112 pages. £2.95. OrderCode BP185.Even relative beginners should find the monophonicsynthesiser described here within their capabilities if thebook is thoroughly read. Individual aspects of the synth aredealt with separately and pcb designs are shown for the mainmodules.


DIGITAL AND COMPUTING

A Concise Introduction to MS-DOS.N. Kantaris. 64 pages. £2.95.Order Code BP232A ready -reference guide for those who need a quick insightinto the essential command functions of this operatingsystem but who don't have the time to (earn it fully.

An Introduction to ComputerPeripheralsR.A. and J.W. Penfold. 80 pages.£2.50. Order Code BPI70Covers such items as monitors, printers, disc drives.cassettes, modems. etc. explaining what they are and how to

use them with your computer and with each other.

Digital Logic Gates andFlip -Flops. Ian R. Sinclair. 192pages. £8.95. Order Code PC 105intelligently looks at the basic building blocks of alldigital circuits and is intended for enthusiasts,students and technicians who seek to establish a firm

grasp of fundamental principles.

Introduction to 6800/6802Microprocessor SystemsR.J.Simpson and T.J.Terrell. 238pages. £10.95. Order Code NT9The book covers systems hardware. programming concepts

and practical experimental work that will assist in

understanding the 6800/6802 microprocessor, with

additional information on the 6802D5E evaluation system.

An Introduction to 68000 AssemblyLanguage.R.A. and J.W.Penfold. 112 pages.£2.95. Order Code BP184Covers the fundamentals of writing programs that will vastlyincrease the speed of 68000 based machines such as theCommodore Amiga, Atari ST range. Apple Mackintosh, etc.

Getting the Most from Your PrinterJ.W.Penfold. 96 pages. £2.95.Order Code BP181How to use the features found on most dot-matrix printersfrom programs and popular wordprocessors. showingexamples of what must he typed to achieve a given effect.

Micro Interfacing CircuitsR.A.Penfold. Two books, each of 112pages.Book 1 : £2.25.Order Code BP130.Book 2 : £2.75. Order Code BP131Both books include practical circuits and useful backgroundinformation though pcb layouts are not included. Book Imainly covers computer input-output techniques. Book 2deals primarily with practical application circuits.

An Introduction to 6502 MachineCode.R.A. and R.W. Penfold. 112 pages.£2.95. Order Code BP147Covers the main principles of machine code programming on6502 -based machines such as the Vic -20. Oric-1/Atmos,Electron, BBC and Commodore 64. It assumes no previousknowledge of microprocessors or machine code and givesillustrative programming examples.

A Z-80 Workshop Manual.E.A.Parr. 192 pages. £3.95.Order Code BPI 12A book for those who already know Basic but wish to exploremachine code and assembly language programming on Z80based computers.

Practical Digital ElectronicsHandbookM.Tooley. 208 pages. £6.95.Order Code PC 104Nine constructional projects introduce digital circuits, logicgates. timers, microprocessors, memory and interfacecircuits - an essential book for anyone interested in digitaldevices.

Computer mProjects ElectronicScienceProjects

GENERAL CONSTRUCTIONALElectronic Science Projects.Owen Bishop. 144 pages. £2.95. OrderCode BP104A bumper bundle of experimental projects ranging incomplexity and including a colour temperature meter,electronic clock, a solid state (led display) scope, an infra -redlaser, a fascinating circuit for measuring the earth's electricalfield strength. and many more.

Electronic Security DevicesR.A.Penfold. 112 pages. £2.50. BP56Full of ideas for keeping your valuables safe. The circuitsinclude designs for light, infra -red. ultrasonic, gas, smoke,flood, door and baby sensors.

More Advanced Electronic SecurityProjects. R.A.Penfold. 112 pages.£2.95. Order Code BP190Follows on from where BP56 leaves off and describes anumber of more up-to-date and sophisticated projects, suchas pyro-sensors. infra -red and doppler-shift detection, fibre -optic loops, and many others.

Electronic Projects for Cars andBoats.R.A.Penfold. 96 pages. £1.95.

Order Code BP9415 fairly simple projects that can be used with a car and/orboat. Stripboard constructional details are included, as areexplanations of the circuit theory.

Power Supply ProjectsR.A.Penfold. 96 pages. £2.50.Order Code BP76A selection of power supply designs. including simpleunstabilised, fixed voltage regulated and variable voltagestabilised. ni-cad charger. voltage step-up, and inverter.

More Advanced Power Supply ProjectsR.A.Penfold. 96 pages. £2.95.Order Code BP192Covers more advanced topics than BP76 and includesprecision supplies, switch mode and computer controlledsupplies, plus a selection of miscellaneous circuits.

Popular Electronic Circuits.R.A.Penfold. 160 pages. £2.95. OrderCode BP80Containing a wide range of circuit designs for experiencedconstructors who are capable of producing working projectsdirect from a circuit diagram without specific constructionaldetails.

DATA AND RMATI N B

Mit Vg Radio and ElectronicsEngineer's Pocket Book - 18thEdition. Keith Brindley. 325pages. £9.95. Order Code NT 11An invaluable compendium of facts, figures andformulae and is indispensable to the designer,student, service engineer and all others interested inradio and electronics.

Rf \972 Microprocessor PocketBook. Steve Money. 252 pages.£9.95. Order Code NT 12Provides a wide selection of information which will beof general use to anyone involved in developing,designing or servicing, or who just wants to learnmore about microprocessor systems.

It'; M Computer Engineer'sPocket Book - 2nd Edition.Michael Tooley. 224 pages.£9.95. Order Code NT 13An invaluable compendium of facts, figures, circuitsand data indispensable to designers, students,service engineers and all others interested incomputer and microcomputer systems.

Op AmpsB.Dance. £7.95.Order CodeNT2Subtitled Their Principles and Applications' this interestingbook is written in a simple non -mathematical style andprovides a source of practical circuits that use bothcommonplace and more sophisticated opamps.

Electronic Hobbyists HandbookR.A.Penfold. 96 pages. £4.95. OrderCode BP233Provides a source of useful information that the amateurenthusiast is likely to need for day-to-day pursuance ofhobby electronics.

Newnes Electronics Pocket BookI.E.Parr. £9.95.Order Code NTIOPresents all aspects of modern electronics in a readableand largely non -mathematical style, and is a good source ofvaluable information for enthusiasts and professionalengineers alike.

Key Techniques for Circuit DesignG.C. Loveday. £6.95.Order Code BM 101Tackles the problems of designing circuits from scratch,introducing the concept of target specifications, the designsequence, device selection, rules of thumb, and usefulequivalent circuits.

HOW TO ORDERState your order code and your name

and address dearly. Enclose a cheque.

PO or international money order (add

50p postage per book -£1.00 for

overseas surface mall). and send to:

PE Book ServiceIntra House193 Uxbridge RoadLondon W12 9RABooks are normally delivered within10 days but please allow 28 days fordelivery.

PRACTICAL ELECIRONICS JULY 1990 59

PE PCB SERVICEIT IS EASY TO BUILD

PRACTICAL ELECTRONICS PROJECTS!

Simplify your project assembly - use a ready-made printed circuitboard. All are fully drilled and roller tinned. Just slot in the componentsas shown in the project texts, and solder them. PCBs are theprofessional route to project perfection.

MAIL ORDERINGSelect the boards you want, and send your order to: PE PCBSERVICE, PRACTICAL ELECTRONICS, 193 UXBRIDGE ROAD,LONDON W12 9RA.

Prices include VAT and postage and packing. Add £2 per board foroverseas airmail. Cheques should be made payable to Intra Press(Payments by Access and Visa also accepted). Quote the project nameand PCB Code Number, and print your name and address in BlockCapitals. Do not send any other correspondence with your order.

TELEPHONE ORDERSUse your Access or Visa card and phone your order to 081-743-8888clearly stating your name and address, card number and order details.All orders receive priority attention. Many PCBs are held in stock, sothey are dispatched within a few days, but please still allow 28 daysfor delivery in case a PCB is temporarily out of stock.

We can only supply the PCBs listed here! Please always check thelatest issue of PE before ordering.

We can also supply the photocopies of the text at £1.50 for eachproject part inclusive of postage and packing ( overseas £2.00).

Please note that we do not supply components - they can be orderedfrom our advertisers!

JUL 87SCOPE STORE oscilloscope add-on data storage148 £11.94SEP 87GCSE TIMER UNIT - versatile variable delay 151 £5.18OCT 87GUITAR TO SYNTH - music interface 157A/B £10.60NOV 87MIDI EXPANDER - Music Interface 159 £5.04DEC 87RS 232C TO MIDI 160 £6.43JAN 88LEGO BUGGY DRIVER 163 £6. 42FEB 88TEACHER TALKBACK - GCSE 164 £6.36DC MOTOR SERVO 165 £7.53MAR 88APPLIANCE TIMER 166A/B £9.38TEACHER LIGHTSHOW - GCSE 167A/B £9.09LOGIC ANALYSER - Double -sided 168 £20.65APR 88LIGHT METAL EFFECTS 169 £7.10JUNE 88AMSTRAD ROM EXPANSION 173 £12.60MAINS MODEM 174 £4.90JULY 88VOCALS ELIMINATOR 175 £4.90AUG 88SPEAKING CLOCK 176 £16.75SEPT 88BBC MULTIPLEXER 177 £4.50OCT 88METAL DETECTOR 178 £6.50DEC 88PANNING MIXER 181 £7.80

MAR 89CAMERA SHUTTER TIMERAPR 89PC MULTIPORTMAY 89KIRLIAN CAMERAJUNE 89

187 £9.95

188A/B £20.55

189A/C £10.50

SOLAR HEATING CONTROLLER 197 £7.20DELUXE METRONOME 198 £10.95JULY 89PROJECTOR SYNCHRONISER 190A £9.50AUG 89EASI-BUILD - VODALEK 191 £5.50HAND CLAPPER 192 £6.50SEP 89EASI-BUILD - COMPRESSOR 193 £5.90FREQUENCY COUNTER -GENERATOR 194A/B £12.50OCT 89EASI-BUILD VOICE -OP -SWITCH 195 £5.90HOME SECURITY CONTROLLER 196A/C £19.50DEC 89VIDEO AGC STABILISER 199 £6.50ECHO STATION 200A/B £11.50MINI METRONOME 201 £5.90JAN 9QBARGRAPH TACHOMETER 202 £5.90EEPROM PROGRAMMER (KEYBOARD VER) 203 £14.50FEB 90EEPROM PROGRAMMER (SWITCH VERSION) 204 £4.90MODEM 205 £11.50MOCK STEREO 206 £4.90MAR 90RADIO CLOCK (TUNER AND PULSE) 207 £6.50APR 90RADIO CLOCK (DISPLAY DECODER) 208 £13.50PC/INTERFACE 209 £5.90MAY 90EPROM POLY-PROG (MAIN PCB) 210 £8.50JUNE 90EPROM POLY-PROG (TOP PCB)MESSAGE MAKERBAUD RATE CONVERTERINTERMITTENT WIPERCIRCUIT BREAKERJULY 90MORSE DECODER

211 £7.50212 £9.50213 £4.90214 £3.90215 £4.90

216 £9.50

PE PCBs are the professional routeto project perfection!


HOW TO DO IT

NICE AND EASY DOES ITBuilding the projects published in PE is a lot easier than someof you perhaps might think. Especially when you use one of

our professionally made printed circuit boards.It's almost like painting by numbers. All the pcbs are fully

drilled, and basically all you need to do is slot in the componentsand carefully solder them to the pcb track pads. Their places areshown in the drawings published with the project.

IDENTITIES

Component identities are usually clearly marked on them.Even if they are colour coded, like some resistors and capacitors,their values are easily worked out from component colour codecharts. From time to time we publish these charts, but it you don'talready have one, send a 9in x 4in stamped and self-addressedenvelope to the Editorial office asking for one.

TOOLS

For many projects you only need a few simple tools -

Soldering iron between 15W and 25W, with a bevelled tip. Dampsponge for keeping the tip clean. Good multicore solder of 18swgor 22swg grade. Fine nose pliers for wire shaping. Adjustablespanner or heavy pliers for tightening nuts. Miniature screwdriverfor adjusting preset controls. Small wire cutters for trimmingcomponent leads. Drill and selection of bits for drilling holes inboxes. Strong magnifying glass for checking joins in close up. It'salso preferable to have a multimeter for setting and checkingvoltages. There are some very good low cost ones availablethrough many of our advertisers, but get one that is rated at aminimum of 20,000 ohms per volt. Many projects do not requireyou to have a meter, but if you are serious about electronics, youreally should have one.

ASSEMBLING THE PCBAuthors will sometimes offer their own advice on the order of

assembly, but as a general guide, it is usually easier to assembleparts in order of size. Start though with the integrated circuitsockets. Please use them where possible, they make life mucheasier than if you solder the ics themselves - with sockets you canjust lift out an is if you want.

Then insert and solder in order of resistors, diodes, presets,small capacitors, other capacitors, and finally transistors. Clip offthe excess component leads after you have soldered them. Nowuse a magnifying glass, ideally one that you can hold to your eye,and take a good look at the joints, checking that they aresatisfactorily soldered, and that no solder has spread between thepcb tracks and other joins. Be really thorough with visualchecking since errors like this are the most likely reason for acircuit not working first time.

SOLDERING

Bring the tip of the iron into contact with the component leadand the pcb solder pad, then bring the end of the solder intocontact with all three, feeding it in as it melts. Once sufficientsolder has melted to fully surround the pad and the lead, removethe solder, and then the iron. Now allow the join to cool beforetouching it, otherwise the solder may set unsatisfactorily. If itdoes move, just reheat the join once more.

WIRING

Connecting the pcb to the various panel controls is the finalassembly stage. Do this just as methodically, following thepublished wiring diagram. You can connect the wires to the pcbin one of three ways. The best is to insert terminal pins into theconnecting holes on the pcb, and then solder wires direct to them.Or, pass the end of the wire through the pcb hole, soldering it onthe other side. Alternatively, the wire can be carefully soldereddirect to the pcb tracking. In all cases first strip the plasticcovering off the wire, twist the strands together, and apply solderto them to keep them secure.

TESTING

Now you are ready to test and use the project as described bythe author. Components can occasionally fail, but these days it isextremely uncommon, and if you have followed the instructions,been careful with your joins, and bought the parts from a goodsupplier, you will have the enormous satisfaction of having builtan interesting and working unit. It really can be easy if you do itwith care.

CHOOSE ONE NOW!

The PE PCB Service list shows all the pcbs available throughPE. Look down the list and see which title takes your fancy -there must be at least one that will interest you! You will probablyalready have the relevant issue of PE, but even if you don't wecan still help you.

BACK ISSUES

We can usually supply copies of back issues of PE up to threeyears old. These are £1.75 each including postage (£2.25 foroverseas readers). If we no longer have the issue needed, we willbe pleased to send a photocopy of the article for the project thatyou want to build. These are £1.00 each per issue, includingpostage (£1.50 to overseas readers).

OBTAINING PARTS

Some projects are available from advertising suppliers ascomplete kits. Otherwise, all the components listed in the textwill be available from suppliers who specialise in individualcomponents.

Occasionally a specific part may only be available from aparticular supplier, if so the source will be given in the parts list.Otherwise there should be no difficulty in buying the parts. Wehave many good suppliers advertising in PE so have a lookthrough their adverts - that's why they're here! Even though apart may not be listed in the adverts, a phone call or two shouldfind a supplier who will be pleased to help. Like us, they too arein the business of encouraging you to enjoy electronics!

PE

WIDE RANGE OF XENON FLASHTUBESWale Phone your anowles

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PLEASE MENTION PRACTICAL ELECTRONICSWHEN REPLYING TO ADVERTS

V

PROGRAMMABLE ELECTRONIC LOCK KITKeys could be a thing ofthe past with this new -highsecurity lock. Secure doorsto sheds, garages, evenyour home or prevent theunauthorised use ofcomputers, burglar alarmsor cars. One 4 -digitsequence will open the lockwhile incorrect entries willsound an alarm. The

number of incorrect entriesallowed before the alarm istriggered is selected byyou. Further entries will be ignored for a time also set by you. Only the correctsequence will open the lock and switch off the alarm. The sequence may easilybe changed by entering a special number and code on the supplied keyboard. Kitincludes: keyboard, alarm buzzer, high quality PCB and all electronic components.Supply: 5 -15V d.c. Will drive our Latch Mechanism (701 150 © £16.50) or relaydirectly.

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XK129 £22.50

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RESISTORS CAPACITORS CAPACITORS CMOS DIODES

Radial Aluminium Electroylitic20% tolerance

10/16/25/35 Volt

4.7,10,22,33,47,100 12p

220,330,470 20p

1000 30p

48p

Disc Ceramic 5% tolerance

150Volt values in pF

15,22,33,47,68 4p

100,150,

220,330,470 6p

10% tolerance value in pF220,330,470,680 6p

4106 3417p p40001N9144001 17p 4160 40p4002 17p 4161 40p4006 37p 4162 40p4007 17p 4163 40p4008 37p 4174 37p4011 17p 4175 40p4012 17p 4194 42p4013 25p 4501 27p4014 37p 4502 40p4115 37p 4503 37p

5p

1N4148 4p

1N4001 3p

1N4002/3/4/5/6 4p

1N5401 7p

1N5402/4/6/8 11p

Loner Diodo6

Carbon Film 0.25W E12 or E24series 1 p each (min quantity 10per value), 80p per 100, £6.00 per

1000.

**Special Offer**Mixed Pack, of 1000 Carbon Film

resistors, 1R to 10M, 100different values all separatelypacked and labeled only £5.90

Metal Film 0.25/0.5 W E96 series

1 OR to 'IMO 4p each (minquantity 10 per value), £3.00 per

100.

Ceramic/wire wound

4W OR1-10K 35p each

7W OR33-12K 37p each

11W 0R68 -10K 40p each

17W 1R -10K 44p each

5% discount on 10+, 10% on 25+

20% on 100+

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100R -10M 20% Horizontal orVertical 24p each. 5% discount on

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20 Turr 3/4" Cermet Pots

10% tolerance 10R -2M 90p each,

5% discount on 10+, 10% on 25+,

20% on 100+

2200

3300 65p

4700 85p

50/63/Volt0.47,1,2.2,3.3,4.7,10 12p

22,33,47 20p

100,220 30p

470 50p

1000 90p

100 Volt

0.47,1,2.2,4.7 12p

10,22 20p

47 30p

100 50p

Ultra miniature Aluminiumelectrolytic radial 20% tolerance4V

220 18p

22,100 18p

16 Volt10,22,47 18p

25 Volt10,22,33 18p

35 Volt4.7,10,22 18p

50 Volt

1000,1500,2200

3300,4700,6800 8p

+80%-20%tol. value pF4700,10000 6p

22000,47000 9,'Tant.Bead resin dipped 20%

tolerance value in uF

6.3 Volt

10,22 15n'47 25p

10 Volt

3.3,4.7,6.8 15p

10,15 25p

22,33,47 35p

16 Volt

2.2,3.3,4.7,6.8 15p

10,15 25p

35"22,33 ,

25 Volt

1,2.2,3.3 15P4.7,6.8 25p

10,15 35" ,

35 Volt

4016 28p 4504 120p4017 37p 4506 76p4018 37p 4508 99p4020 37p 4510 37p4021 37p 4511 37p4022 37p 4512 37p4023 17p 4513 99p4024024

5 17p35p 4514 85p

4515 80p4027 34p 4516 37p4028 37p 4517 99p4029 37p 4518 37p4032 56p 4519 26p4034 95p 4520 37p4035 44p 85p4038 65p 4522 44p4040 37p 4526 44p4042 37p 4527 44p4043 37p 4528 44p4044 37p 4529 50p

4530 99p4046 47p4049 27p 4531 44p4050 27p 4532 60p4051 37p 4534 240p4052 37p 4536 120p4053 37p 4538 54p4060 37p 4539 45p4066 29p 4541 50p4067 99p 4543 54p4068 170 4544 1300

4069 17p 4547 130p4070 17p 4549 400p4071 17p 4551 85p4072 17p4073 17p

4553 1200

4554 320p4075 170 4555 50p4076 37p 4556 50p4077 17p 4557 12004078 17p 4558 120p4081 17p 4559 440p4082 17p 4560 110p

500 mw

2.4,2.7,3.3,3.9,4.7,5.1,5.6,6.2,6.8,8.2,10,11,12,13,15,16,18,20,22 24 27 30 33 39 43 62 66 75 8291

All above voltages at 5p each

1.3W

3.3, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2,6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13,

15, 16, 18, 20, 22, 24, 27, 30, 33,36, 39, 43, 47, 51, 56, 62, 68, 75,

82, 91.

All above voltages at 16p each

2.5W,20W & 75W versions

available

Bridges

W005 23p

W02 25p

W04 30p

W06/08 35p

2A 200V 65p2A 400V 72p2A 600V 9006A 200V 78p6A 400V 8406A 600V 99p

0.1,0.22,0.33,0.47, 18p

1,2.2,3.3,4.7,10

0.1,0.22,0.33,0.47 15P0.68,1,2.2,3.3 20p

4.7,6.8,10 30p

4093 27p call sales for 45004094 48p series above 45604097 99p4099 46p

Linear I.C.s call forprice

TRANSISTORSKITS KITS KITSBC107B 20p

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The above are a few examplesfrom the 1000's of transistortypes we have. We cover allranges and types including JFET,

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ALL KITS ARE SUPPLIED WITH HIGH QUALITY GLASS FIBRE SILK SCREENEDPCB, FULL INSTRUCTIONS, HEATSINK, SOLDER ETC. CASE NOT SUPPLIED BUTAVAILABLE.CONSTANT CURRENT NI -CAD CHARGER Charge your NI -Cads safely, can be left on idenfinitely withoutdamage, batteries fully charged from flat in approx. 18 hours. Charge up to 12 batteries in series (except PP3). (withtransformer) £7.50FAST NI -CAD CHARGER Rapidly charge your NI -Cad racing pack from mains or 12Volts. Charger guarantees a

full charge every time, making maximum use of your pack. (with transformer) £10.00BENCH POWER SUPPLY 2.5 - 35 Volt @ 2 Amps The following spec ensures excellent value formoney for this most essential of workshop equipment. Line Reg. 0.03%, Load Reg. 0.1% Ripple Rejection 70 dB Output

Res. 1.5 milliOhm, Output Noise 80uV. (with transformer) t8.0010 Amp BENCH POWER SUPPLY 5 - 33 Volt Use for powering C.B/Ham Radios, or a very highpowered bench power supply. Spec as 2A version above. (with transformer) £32.00FM Bug, 26 x 16 x 12mm, super sensitive 500yd range 4.5-9V supplied with miniature mic & ready wound coil.

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REGULATORS7805 5V 01.5A 30p

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Does yours pass the screen test?We believe ours do!!!Precision laboratory oscilloscopes. Triple -trace 20MHz 3 channels -3 trace. XY mode allows Lissajous patternsto be produced and phase shift measured. 150mm rectangular CRT has internal graticule to eliminate parallaxerror. 2Ons/div sweep rate makes fast signals observable. Stable triggering of both channels even withdifferent frequencies is easy to achieve and a TV sync separator allows measurement of video signals.Algebraic operation allows the sum or difference of channel 1 and 2 to be displayed. 50mV/div output from CH1 available to drive external instrument e.g. frequency counter. Also available, 40MHz triple trace oscilloscope.Similar to the model described above but with 12kV tube that is super bright even at the highest frequencies.This instrument also has a delayed sweep time base to provide magnified waveforms andaccurate time interval measurements.

TOA10 (20MHz Triple Scope) £334.95TOB10 (40MHz Triple Scope) £549.95

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.1461.COMMUNICATIONS PROJECT I-i-T*module whether it is to regard codes on itsdata lines as character addresses or screencontrol data. The E line can be regarded as theenable or clock line, and this has to be takenup and down each time a new instruction codeis to be actioned.

In this morse decoder we use the moduleunder 8 -bit data control, with one screen line,shifting left by one place each time the E lineis toggled. Each new character is placed atscreen position eight, ie, the far right handside. The effect is that the decoded morsemessages appear to be travelling in from theright and out at the left of screen. Noprovision has been allowed with the decoderfor recalling data that has moved off screen.

INITIALISATIONGATING

The eprom is programmed so that the setof instructions required for initialisation isstored in an area separate from the normalcharacter decoding codes. Eprom line A9when high re -addresses the eprom to theinitialisation codes area. With lines A3 -A8,plus A10, held low, the counter IC4a isclocked from a separate oscillator so that theeprom progressively sends the requiredinitialisation codes in sequence. At the end ofthe routine the system automatically switchesitself to morse-receive mode.

IC5c and IC5d form the oscillatorresponsible for clocking the counter and thelcd module in the initialisation routine. Theclock frequency is set by C12 and R14,though the actual frequency is largelyirrelevant, providing it is within audio range.

At switch -on, C6 begins to charge via R8.Both components are connected to the clackinput of the counter IC4b. When the voltagelevel on C6 has risen sufficiently, IC4b istriggered and its QO output goes high. Thissets eprom address line A9 high, so switchingthe eprom's memory to the routinescontrolling initialisation.

QO is inverted by IC5f which takes the. RSline low, so telling the module that anyinstructions coming in on the data lines DO -D7 are to be treated as control commandsrather than screen data information.

The output from IC5f also controls thegate IC6. Internally, the gate consists of fourseparate single pole changeover switches. Theblock diagram is shown in Fig.4. With IC6'sSelect input, pin 1, held low by IC5f, the gateis switched so that inputs 2, 5, 11 and 14 arerouted to outputs 4, 7, 9 and 12 respectively.Consequently, output pin 4 holds IC3 resetand momentarily resets IC4a. The oscillator isrouted to the clock input of IC4a, and also tothe E line.

The oscillator clocks IC4a step by step,causing the eprom to progressively cyclethrough the data held as instructions for thelcd. Since the lcd is simultaneously triggeredby the oscillator, it responds to theseinstructions, putting itself into the requiredoperational mode referred to above. There areonly eight instructions required so on theninth count IC4b is reset by the high level

LOGIC DIAGRAM

NIBBLE

A INPUTS

2n

Al

11A2

40

7

00

Y13

DATABD

6 9Y2

NIBBLEOUTPUTS

to

10 123B INPUTS B2

83

SELECT

15OUTPUT

ENABLE

PIN 16=Vc6PIN B=GND

Fig 4. Logic diagram of the 74 HC157chip used for 106.

now on IC4a pin 14. This action terminatesthe initialisation sequence since IC4b QO issimultaneously reset to zero.

Gate IC6 automatically switches over to itsother state, now routing input pins 3, 6, 10and 13 to outputs 4, 7, 9 and 12 respectively.This condition remains stable as IC4b cannotbe retriggered by the clock input because C6will remain fully charged until the unit isswitched off. It is in this stable state of IC6that the signal routing described in the bodyof the text takes place.

TONE OUTPUTYou will notice that I have provided an

option for sending a constant tone via C20and R15 to an external switch, S2, and avolume control VR4. I didn't build this optioninto my own box, but I have placed C20 andRI5 on the pcb. The intention of the option isallow for a morse practice key to be used asS2. By coupling the output of VR4 back to thesignal input at CI, you can practice yourmorse code and see the accuracy of yourkeying displayed on the lcd screen.

POWER SUPPLYThe circuit requires a stabilised 5V supply.

The limit of 5V must not be exceededotherwise the eprom, the lcd module and IC6will die rather hurriedly. The only way youcan safely run the unit from a supply greaterthan 5V is to use the optional regulator chipIC8. Voltages up to around 9V dc can be putinto IC8 and it will then deliver a stable 5Vsupply to the rest of the circuit.

The lcd module additionally requires anegative voltage supply. This is generated byusing the oscillator as the drive for an invertercircuit. C13 -C15, D7, D8 and VR5 are theassociated components. Since the negativevoltage determines the lcd's screen contrast,VR5 is included to preset it to the desiredlevel, usually at around -3V, though this willdepend partly on ambient light conditions.

000 20 45 49 53 48 35 ff ff008 ff ff ff ff ff ff ff ff010 ff ff ff ff ff 36 ff ff018 ff ff ff ff ff ff ff ff020 ff ff ff ff 42 26 ff ff028 ff ff ff ff ff ff ff ff030 ff ff ff ff ff 37 ff ff038 ff ff ff ff ff ff 3B ff040 ff ff ff 44 4c 65 ff ff048 ff ff ff ff ff ff ff ff050 ff ff ff ff ff ff ff ff058 ff ff ff ff ff ff ff ff060 ff ff ff ff 5a ff 3f ff068 ff ff ff ff ff ff ff ff070 ff ff ff ff ff 38 ff ff078 ff ff ff ff ff ff ff ff080 ff ff 4e 52 46 23 ff ff088 ff ff ff ff ff ff ff ff090 ff ff ff ff ff 2f 3c ff098 ff ff ff ff ff ff ff ffOAO ff ff ff ff 43 2b ff ff0A8 ff ff ff ff ff ff ff ffOBO ff ff ff ff ff ff ff ffOB8 ff ff ff ff ff ff ff ffOCO ff ff ff 47 50 ff ff ff008 ff ff ff ff ff ff ff ffODO ff ff ff ff ff 28 ff ffOD8 ff ff ff ff ff ff ff ff0E0 ff ff ff ff 6f ff ff ff0E8 ff ff ff ff ff ff ff ffOFO ff ff ff ff ff 39 27 ffOF8 ff ff ff ff ff ff ff ff100 ff 54 41 55 56 34 ff ff108 ff ff ff ff ff ff 2d ff110 ff ff ff ff ff 3d ff ff118 ff ff ff ff ff ff ff ff120 ff ff ff ff 58 ff ff ff128 ff ff ff ff ff ff ff ff130 ff ff ff ff ff ff ff ff138 ff ff ff ff ff ff ff ff140 ff ff ff 4b 61 ff 2f ff148 ff ff ff ff ff ff ff ff150 ff ff ff ff ff 2b 2e ff158 ff ff ff ff ff ff ff ff160 ff ff ff ff 51 61 21 ff168 ff ff ff ff ff ff 29 ff170 ff ff ff ff ff ff ff ff178 ff ff ff ff ff ff ff ff180 ff ff 4d 57 75 33 ff ff188 ff ff ff ff ff ff ff ff190 ff ff ff ff ff ff ff ff198 ff ff ff ff ff ff 2c ff1A0 ff ff ff ff 59 ff ff ff1A8 ff ff ff ff ff ff ff ff1B0 ff ff ff ff ff 6e ff ff1B8 ff ff ff ff ff ff ff ff1C0 ff ff ff 4f 4a 32 ff ff1C8 ff ff ff ff ff ff ff ff1D0 ff ff ff ff ff ff ff ff1D8 ff ff ff ff ff ff ff ff1E0 ff ff ff ff 2e 31 ff ff1E8 ff ff ff ff ff ff ff ff1F0 ff ff ff ff ff 30 ff ff1F8 ff ff ff ff ff ff ff ff200 30 30 30 34 Oc 01 07 88200 to 3f8 : don't care (not used)

400 a5 a5 a5 a5 a5 a5 a5 a5then repeat a5 till:500 b0 b0 b0 b0 b0 b0 b0 b0then repeat b0 till:600 30 30 30 34 Oc 01 07 88

Fig 5. (E)eprom hex dump.


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PRACTICAL JULY 1990 £1.50 ELECTRONICS