FM WIRELESS MICROPHONE KIT

MODEL K-30/AK-710

Assembly and Instruction Manual

Elenco® Electronics, Inc.Copyright © 2006, 1994 by Elenco® Electronics, Inc. All rights reserved. Revised 2006 REV-J 753016No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission from the publisher.

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PARTS LISTIf you are a student, and any parts are missing or damaged, please see instructor or bookstore.If you purchased this kit from a distributor, catalog, etc., please contact Elenco® Electronics (address/phone/e-mail is at theback of this manual) for additional assistance, if needed. DO NOT contact your place of purchase as they will not be ableto help you.

RESISTORSQty. Symbol Value Color Code Part #

1 R5 150Ω 5% 1/4W brown-green-brown-gold 1315002 R8, R10 1kΩ 5% 1/4W brown-black-red-gold 1410001 R7 1.5kΩ 5% 1/4W brown-green-red-gold 1415001 R3 4.7kΩ 5% 1/4W yellow-violet-red-gold 1447001 R1 8.2kΩ 5% 1/4W gray-red-red-gold 1482001 R6 10kΩ 5% 1/4W brown-black-orange-gold 1510001 R2 27kΩ 5% 1/4W red-violet-orange-gold 1527002 R4, R9 47kΩ 5% 1/4W yellow-violet-orange-gold 154700

CAPACITORSQty. Symbol Value Description Part #

1 C4 10pF (10) Discap 2110111 C5 12pF (12) Discap 2112101 C6 33pF (33) Discap 2133172 C3, C7 .001µF (102) Discap 2310352 C1, C2 .1µF (104) Discap 251010

SEMICONDUCTORSQty. Symbol Value Description Part #

3 Q1 - Q3 2N3904 Transistor 3239041 LED Light Emitting Diode (LED) 3500011 Coil FM Mic 468751

MISCELLANEOUSQty. Description Part #

1 PC Board 5177101 Switch (S1) 5410241 Mic 5680001 Battery Clip (+) 5900911 Battery Clip (–) 5900931 Foam Cover 6200021 Top Case 6231051 Bottom Case 6232051 Stand 626010

Qty. Description Part #1 Battery Cover 6270021 Alignment Tool 6290111 Screw 2.5mm x 4mm 6413103 Screw 2.6 x 8mm 64210912” Wire 22ga. Gray 8148106” Wire 26ga. Black 81621010.5” Wire 26ga. Red 8162201 Solder Tube 9ST4

Resistor

PARTS IDENTIFICATION

FM Coil

Microphone

Capacitor Transistor

Battery Clips

(–)(+)

SwitchLED

Case

Stand Battery Cover

Top

Bottom

Caution: Do not mix alkaline, standard (carbon-zinc), or rechargeable (nickel-cadmium) batteries.

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FM MICROPHONE KITYour FM Microphone is really a miniature frequencymodulated transmitter operating in the standard FMbroadcast band. The range of frequencies for theFM broadcast band is 90MHz (MHz = Megahertz or90 million cycles per second). Because the FMmicrophone has a variable tuned circuit, it can betuned to a quiet spot on your local FM broadcastband for the best reception. When the small

microphone element is struck by sound, it convertsthe audio to a change in current through resistor R1(see schematic diagram). This electrical change isamplified and eventually frequency modulates thetransmitter. The transmission range of the FMmicrophone is approximately 100 feet, dependingon the efficiency of the antenna (properly tuned ornot) and the quality of the FM radio receiver.

IDENTIFYING RESISTOR VALUESUse the following information as a guide in properly identifying the value of resistors.

BAND 11st Digit

Color DigitBlack 0Brown 1Red 2Orange 3Yellow 4Green 5Blue 6Violet 7Gray 8White 9

BAND 22nd Digit

Color DigitBlack 0Brown 1Red 2Orange 3Yellow 4Green 5Blue 6Violet 7Gray 8White 9

Multiplier

Color MultiplierBlack 1Brown 10Red 100Orange 1,000Yellow 10,000Green 100,000Blue 1,000,000Silver 0.01Gold 0.1

ResistanceTolerance

Color ToleranceSilver +10%Gold +5%Brown +1%Red +2%Orange +3%Green +0.5%Blue +0.25%Violet +0.1%

BANDS 1 2 Multiplier Tolerance

IDENTIFYING CAPACITOR VALUESCapacitors will be identified by their capacitance value in pF (picofarads), nF (nanofarads), or µF (microfarads). Mostcapacitors will have their actual value printed on them. Some capacitors may have their value printed in the followingmanner. The maximum operating voltage may also be printed on the capacitor.

Second Digit

First Digit

Multiplier

ToleranceThe letter M indicates a tolerance of +20%The letter K indicates a tolerance of +10%The letter J indicates a tolerance of +5%

For the No. 0 1 2 3 4 5 8 9

Multiply By 1 10 100 1k 10k 100k 0.01 0.1Multiplier

Note: The letter “R” may be used at timesto signify a decimal point; as in 3R3 = 3.3

103K100V

Maximum Working Voltage

The value is10 x 1,000 =10,000pF or.01µF 100V

10µF 16V

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BASIC MODULATION THEORYThere are many different methods for modulatinginformation onto a radio wave. The two mostpopular methods are Amplitude Modulation (AM)and Frequency Modulation (FM). Figure 1 showsthe basic difference between these two methods. Inan amplitude modulated radio wave, the audioinformation (voice) varies the amplitude of the RFcarrier. To recover this information, all that isneeded is a peak detector that follows the carrierpeaks. This is fairly easy to understand. In afrequency modulated radio wave, the informationchanges the frequency of the carrier as shown inFigure 1.

The amplitude of the radio frequency carrier waveremains constant. The loudness of the audiodetermines how far the frequency is moved from theunmodulated carrier frequency. In a normal FMradio broadcast, the maximum deviation from centerfrequency is set at +150kHz for the loudest sound.A soft sound may move the carrier only +10kHz.The number of times the carrier deviates from thecenter frequency, each second depends on thefrequency of the audio. For example, if the carrier ismoved to +75kHz, then –75kHz 1,000 times eachsecond, the carrier is 50% modulated for loudnesswith a 1,000 cycle audio tone.

One advantage of FM modulation over AMmodulation is the carrier amplitude is not importantsince the information is carried by the frequency.This means that any amplitude noise added to thesignal after transmission (such as lightning, spark orignition noise in cars, etc.) can be reduced byallowing the amplifiers before detection to limit orsaturate. This principle is shown in Figure 2.

The standard broadcast band for FM was alsodesigned to have an audio range up to 25,000 Hertz(Hertz = cycles per second). The standard AM

broadcast band has only 7,000 Hertz band width(Figure 3). The FM band is therefore considered tobe “High Fidelity” compared to the older AM band.

Another big advantage that FM has over AM is the“Capture” effect in FM broadcast. If two differentbroadcasts are very close in frequency or on thesame frequency in AM, they will produce an audiotweet or beat. In FM, the receiver will “Capture” thestrongest signal and ignore the weaker one. Inother words, if a local transmitter and anotherdistant transmitter are on the same frequency, theFM receiver will lock in on the strong local stationand reject the weak one. In an AM radio, if the sameconditions exist, you will hear a beat (a whistle)between the two stations, which is very annoying.

Capture works because the receiver “sees” radiowaves as the sum of each frequency present. SinceFM only looks at frequency, the weaker signal canbe eliminated by the limiter as shown in Figure 4.The detector “sees” only the strong signal after thelimiting amplifier has stripped the weak one away.

Amplitude Modulation

Frequency Modulation

Figure 1

Figure 2

Original TransmittedSignal

Received Signal withNoise and Fading

Received SignalAfter Limiting

Amplifier

Figure 3

Narrow Band

WideBandwidth

7kHz 25kHz

AM Broadcast Band FM Broadcast Band

Audio Bandwidth for AM & FM

Figure 4

Two FrequenciesTransmitted What Limiter “sees” Capture Effect

Output from limiterF1 only . . . F2 removed

F1 F2 F1 + F2 Limiter Levels

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CIRCUIT OPERATIONFigure 5 shows a block diagram of the FM wirelessmicrophone circuit. The microphone element inBlock 1 acts like a resistor that changes whenexposed to sound waves. The change in resistancecauses current through the microphone element tochange when sound waves apply pressure to itssurface. This action is similar to squeezing a gardenhose and watching the water through it decrease.When the hose is released, the water through it willincrease. When sound waves hit the microphoneelement, the electrical current through the elementwill increase and decrease according to the pressure(loudness) of the sound.

Block 2 is a transistor (Q1) used as an audioamplifier. The signal from the micro-phone elementis increased in amplitude by a factor of 3. Inelectronics, this action is described as transistor Q1having an audio gain of 3.

Block 3 is a transistor (Q2) used as an oscillator. Anoscillator is an electronic circuit similar to thependulum in a grandfather clock. Once thependulum is started in motion, it will use only asmall amount of energy from the main spring tokeep it swinging at the exact same frequency. It isthis stable frequency rate that sets the timeaccurately. If the weight is moved down the stick onthe pendulum, the swing takes longer if thefrequency is lower. If the weight is moved up thestick, the frequency increases. This is called tuningthe frequency of the pendulum. In electronics, anoscillator circuit also has tunable elements. Theinductor in a tuned circuit is equivalent to the lengthof the pendulum (see Figure 6).

By changing the position of the iron core in theinductor, the inductance can be changed to tune theoscillator to a desired radio frequency, just likechanging the weight of the pendulum would changeits frequency. When sound strikes the microphoneelement, it is converted to an electrical signal,amplified and used to change the capacitance(length of the pendulum) of the electronic oscillator’stuned circuit. This causes the frequency of theoscillator to make slight changes at the same rateas the sound striking the microphone. This effect isknown as frequency modulation.

Block 4 is a transistor used as a radio frequencyamplifier. This block amplifies the modulated signalfrom the oscillator and acts as a buffer stagebetween the antenna and the oscilator. If theantenna were tied directly to the oscillator withoutthe buffer, any capacitance added to the antenna(touching it with your finger for example) wouldproduce a large change in the frequency ofoscillation. The receiver would not be able to followthis large change in frequency and would lose thetransmission.

Block 5 is the antenna. The antenna is also a tunedelement since the length of the antenna determineshow well it will radiate the modulated signal. Anantenna acts much like a piece of string tied to awall and stretched tight. If you tap the string, a wavewill travel to the wall and part of the energy will gointo the wall and part will be reflected back (seeFigure 7A). If the length of the string is adjusted tomatch the rate of tapping as shown in Figure 7B, thewall receives all of the energy because it is at anode or proper multiple of the wavelength. Inelectronics, the wall is similar to the space aroundthe antenna. By properly tuning the antenna, all ofthe available power in the antenna will be radiatedinto the space around the antenna. None will reflectback. A term used in electronics to describe theamount of power reflected back as a ratio of theamount of power radiated is called “The StandingWave Ratio”.

Figure 5

MicrophoneElement

Q1 Q2 Q3

AudioAmplifier

RadioFrequencyOscillator

RadioFrequencyAmplifier

Antenna

Block 5Block 4Block 3Block 2Block 1

Figure 6

Length

Weight

CL

C = CapacitanceL = Inductance

Pendulum Electronic TunedCircuit in Oscillator

Figure 7B

Figure 7A

Original Wave Reflected Wave

Wall

All energy goesinto the wall,none is reflected.

Vibrating String Node

IntroductionThe most important factor in assembling your FM Wireless Microphone is good soldering techniques. Using theproper soldering iron is of prime importance. A small pencil type soldering iron of 25 - 40 watts isrecommended. The tip of the iron must be kept clean at all times and well tinned.

Safety Procedures• Wear eye protection when soldering.• Locate soldering iron in an area where you do not have to go around it or reach over it.• Do not hold solder in your mouth. Solder contains lead and is a toxic substance. Wash your hands

thoroughly after handling solder.• Be sure that there is adequate ventilation present.

Assemble ComponentsIn all of the following assembly steps, the components must be installed on the top side of the PC board unlessotherwise indicated. The top legend shows where each component goes. The leads pass through thecorresponding holes in the board and are soldered on the foil side.Use only rosin core solder of 63/37 alloy.

DO NOT USE ACID CORE SOLDER!

CONSTRUCTION

Solder Soldering Iron

Foil

Solder

Soldering Iron

Foil

Component Lead

Soldering Iron

Circuit Board

Foil

Rosin

Soldering iron positionedincorrectly.

Solder

GapComponent Lead

Solder

Soldering Iron

DragFoil

1. Solder all components fromthe copper foil side only.Push the soldering iron tipagainst both the lead andthe circuit board foil.

2. Apply a small amount ofsolder to the iron tip. Thisallows the heat to leave theiron and onto the foil.Immediately apply solder tothe opposite side of theconnection, away from theiron. Allow the heatedcomponent and the circuitfoil to melt the solder.

1. Insufficient heat - thesolder will not flow onto thelead as shown.

3. Allow the solder to flowaround the connection.Then, remove the solderand the iron and let theconnection cool. Thesolder should have flowedsmoothly and not lumparound the wire lead.

4. Here is what a good solderconnection looks like.

2. Insufficient solder - let thesolder flow over theconnection until it iscovered. Use just enoughsolder to cover theconnection.

3. Excessive solder - couldmake connections that youdid not intend to betweenadjacent foil areas orterminals.

4. Solder bridges - occurwhen solder runs betweencircuit paths and creates ashort circuit. This is usuallycaused by using too muchsolder. To correct this,simply drag your solderingiron across the solderbridge as shown.

What Good Soldering Looks LikeA good solder connection should be bright, shiny,smooth, and uniformly flowed over all surfaces.

Types of Poor Soldering Connections

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ASSEMBLE COMPONENTS TO THE PC BOARD

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C4 - 10pF Discap (10)

Q2 - 2N3904 Transistor(see Figure A)

L1 - Coil

C5 - 12pF Discap (12)

C6 - 33pF Discap (33)

R8 - 1kΩ 5% 1/4W Res.*(brown-black-red-gold)

C7 - .001µF Discap (102)

R10 - 1kΩ 5% 1/4W Res.*(brown-black-red-gold)

R1 - 8.2kΩ 5% 1/4W Res.(gray-red-red-gold)

R4 - 47kΩ 5% 1/4W Res.(yellow-violet-orange-gold)

R3 - 4.7kΩ 5% 1/4W Res.(yellow-violet-red-gold)

C1 - .1µF Discap (104)

R2 - 27kΩ 5% 1/4W Res.(red-violet-orange-gold)

R6 - 10kΩ 5% 1/4W Res.(brown-black-orange-gold)

R5 - 150Ω 5% 1/4W Res.*(brown-green-brown-gold)

R9 - 47kΩ 5% 1/4W Res.*(yellow-violet-orange-gold)

Q3 - 2N3904 Transistor(see Figure A)

C3 - .001µF Discap (102)

C2 - .1µF Discap (104)

Q1 - 2N3904 Transistor(see Figure A)

R7 - 1.5kΩ 5% 1/4W Res.(brown-green-red-gold)

Mount thetransistorwith theflat sideas shownon the toplegend.

Figure A

Mounttheseresistorson end.

*Top Legend of PC Board

Strip the insulation off of one endof the 12” gray wire to expose 1/8”of bare wire. Mount and solder thewire to the foil side of the PC boardin hole J5.

Cut a 1 1/2” red wire and 1 1/2”black wire. Strip the insulation offof both ends to expose 1/8” of barewire. Mount and solder the redwire to the foil side of the PC boardin hold J6 (+) and the black wire tohole J7 (–).

Cut the leads of the LED so thatthey are 1/4” long, then spreadthem slightly apart (see Figure B).

Solder the free end ofthe black wire to theflat side lead of theLED. Solder the freeend of the red wire tothe other lead of theLED.

Figure B

If your microphone has leads attachedto it, cut them off flush with the pads onthe microphone. Cut a 2” piece of redwire and a 2” piece of black wire. Stripthe insulation off of both ends to expose1/8” of bare wire. Solder the red wire tothe foil side of the PC board in hole J1(+) and the black wire to hole J2 (–).

Solder the free end of the red wire tothe (+) pad on the mic and the blackwire to the (–) pad on the mic as shownin Figure C.

Figure C+

J7 J2

J1J6

J5

Black

Red

Foil Side of PC Board

Red

Black

Flat

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Mount the slide switch onto the foil sideof the PC board. The tabs on the switchmust go through the slots of the PCboard. Solder the switch to the PCboard.

Strip the insulation off of both ends onthe remaining 2 1/2” of black wire and the7” of red wire to expose 1/8” of bare wire.Mount and solder the black wire to thefoil side of the PC board in hole J4 (–)and the red wire in hole J3 (+).

Solder the free end of the black wire tothe negative (–) clip.

J3

J4

Foil Side of PC Board

Insert the free end of the red wirethrough the slot in the bottom caseand solder to the positive (+) clip(see Figure D).

Pull the wire back through the slot andinsert the clip into the case and bendthe tab as shown in Figure E.

(+)(–)

Foil Side

Figure D

Red Wire

Figure E

Pull wire down

Bend tab

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Insert the mic into the slot as shown in Figure F.Insert the LED into the hole as shown in Figure F.Place a piece of tape over the LED to hold it inplace.Insert the PC board into the top case, as shownin Figure G.Insert the negative (–) battery clip into the bottomcase as shown in Figure G.Press the gray antenna wire and the 7” piece ofred wire through the slots in the top case asshown in Figure G.

Place the bottom case onto the top case. Holdthe two halves together with three 2.6 x 8mmscrews and one 2.5mm x 4mm screw, as shownin Figure H.Insert the stand in the case as shown in Figure H.Push the foam cover onto the case as shown inFigure H.Insert two “AA” batteries into the case with thepositive (+) side toward the back end of the case(see the inscription on the inside of the case).Caution: Do not mix alkaline, standard (carbon-zinc), or rechargeable (nickel-cadmium)batteries. Insert the battery cover onto the caseas shown in Figure H.

FINAL ASSEMBLY

Figure F

Mic LED

Figure H

Foam Cover

2.6 x 8mm Screws

2.6 x 8mm Screws

Battery Cover

Stand

2.5 x 4mm Screw

Figure G

Gray Antenna Wire

Red WireBlack Wire

Bottom Case

Top Case

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Tug slightly on all parts to make sure that they areindeed soldered.

A solder bridge may occur if you accidently touchan adjacent foil by using too much solder or bydragging the soldering iron across adjacent foils.Break the bridge with your iron.

Make sure that all of the parts are placed in theircorrect position. Check if the transistors’orientations are correct.

Make sure that the polarity of the LED andmicrophone are placed in the correct position.

FOIL SIDE OF PC BOARD

OPERATING THE FM MICAfter assembling the kit, it will be necessary to tunein the transmitter. First, be sure that all of the partsare in correctly and that you have good solderconnections without any solder shorts.

Get an FM radio and tune it away from any FMstations. You should hear only noise and noprograms. Place the unit about 2 feet from the radiowith the microphone facing the radio speaker.Remove the foam cover and tune the RF coil withthe tuning stick and listen for a howl in the radio.This indicates that you have tuned the transmitter tothe FM radio frequency. Place the transmitter awayfrom the radio until the howl disappears. Talk into

the microphone and you should hear your voice onthe radio. If your voice comes through the radiodistorted, speak softer (you are over-modulating).Push the foam cover onto the case when tuned.

Have a friend listen to the radio and move thetransmitter about 100 feet away. Your voice shouldstill be heard over the radio. To obtain furtherdistance, add a longer antenna.

TO CONSERVE BATTERIES, TURN THE POWERSWITCH OFF WHEN NOT ACTUALLY TALKING.

TROUBLESHOOTING

SCHEMATIC DIAGRAM

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GLOSSARYAmplitude Modulation To modify the amplitude of the carrier

wave in accordance with the desiredsignal, often abbreviated as AM.

Amplify To enlarge; increase in scope orvolume.

Antenna A device by which radio waves arereleased or received.

Audio Sounds that are capable of beingheard by the human ear.

Buffer Stage A circuit used to insulate signalsfrom other circuits.

Capacitor A device that is capable of holdingan electric charge.

Capture The capacity of an FM receiver topick only the strongest signal andthus reduce co-channelinterference.

Carrier Wave The unmodulated wave radiated bya broadcast station.

Deviation The change in frequency away fromthe carrier wave due to FMmodulation.

Efficiency The ratio of energy expended topower produced.

Frequency Modulation To modify the frequency of the carrierwave in accordance with the desiredsignal, often abbreviated as FM.

FM Broadcast Band The range of frequency wherecommercial frequency modulationis allowed by the FederalCommunications Commission(FCC).

FM Transmitter The sending apparatus of a radiowave in which the message iscontained in the frequency of thecarrier wave.

Hertz A term used to indicate the numberof cycles per second.

High Fidelity A term used to indicate totalcoverage of the hearing system.

Inductor A device capable of storingelectrical energy in the form of amagnetic field.

MHz or Megahertz A million cycles per second.

Microphone A device used for producing anelectrical current corresponding inits variations in air pressure ofsound.

Modulate To modify a characteristic of acarrier wave in accordance with thecharacteristics of a desired signal.

Noise In electronics, noise is usually therandom electrical signal producedby the thermal agitation of atoms orstatic discharges.

Oscillator A device that continually swingsback and forth between two fixedpoints.

Peak Detector A device used to recover themodulated signal from an amplitudemodulated wave.

Pendulum A body suspended from a fixedpoint so that it may swing freely.

Resistor An electric device used to restrictthe flow of electrical current.

RF Carrier The radio frequency wave used to“carry” the desired signal.

Saturate Completely charged or at its limit ofoperation.

Standing Wave Ratio A term used in electronics todescribe the amount of powerreflected back as a ratio of theamount of power radiated.

Tuned Circuit A collection of components used toselect a single or small group offrequencies.

QUIZ1. The letters FM stand for ___________ ___________.

2. In AM transmissions, the audio information varies the _____________ of the radio frequency carrier wave.

3. In FM transmissions, the audio information varies the _____________ of the radio frequency carrier wave.

4. In a standard FM radio broadcast moving the carrier +75kHz from the center frequency would represent_____________% modulation.

5. The effect of rejecting the weaker station and accepting only the strong station is called ______________.

6. When the microphone is exposed to sound waves, it acts like a changing _____________.

7. An oscillator circuit is similar to the _______________ in a clock.

8. Sound striking the microphone is converted to an electrical signal, amplified, and used to change the_____________ of the electronics oscillators tuned circuit.

9. Using an element to change the frequency of an oscillator at the same rate as the data to be transmitted iscalled ____________ ____________.

10. The antenna is also a _____________ element.

Answers:1.frequency modulation;2.amplitude;3.frequency;4.50%;5.capture;6.resistor;7.pendulum;8.capacitance;9.frequency modulation;10.tuned

Elenco® Electronics, Inc.150 Carpenter AvenueWheeling, IL 60090

(847) 541-3800Web site: www.elenco.come-mail: elenco@elenco.com