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Use it to rebroadcast video signalsthroughout your house.

BY MARC SPIWAK

One of the most useful gadgets a videoenthusiast can have is a low-power TVTransmitter. Such a device can transmit asignal from a VCR to any TV in a home orbackyard. Imagine the convenience ofbeing able to sit by the pool watching yourfavorite movie on a portable with a tape orlaserdisc playing indoors. You could evenretransmit cable TV for your own privateviewing. Videotapes can be dubbed from

one VCR to another without a cableconnecting the two machines together.When connected to a video camera, a TVtransmitter can be used in surveillance formonitoring a particular location.

The main problem a video enthusiasthas in obtaining a TV transmitter is that acommercial units are expensive. However,we have some good news! You can buildthe TV Transmitter described here for less

SEP 1

5 1997

TVTransmitter

P O P t r o n i x T V T r a n s m i t t e r w w w . p o p t r o n i x . c o m2

than $30 in one evening! The easiest wayto do that is to order the kit that‚s availablefrom the source given in the Parts List (acustom case for the kit is also available).Nevertheless, we present enoughinformation here to build the TVTransmitter from scratch.

The TV Transmitter combines line-level audio and video signals, andtransmits the resulting signal up to 300feet. The circuit can be powered from a 9-volt battery. It is suggested that a 12-voltDC supply during be used during thealignment procedure. This would insuremaximum transmission range and bestpossible picture. Aligning the TVTransmitter requires no special equipmentwhatsoever, and it is a very simpleprocedure. The Transmitter's output canbe tuned to be received on any TV channelfrom 2 to 6. The range of channels is wide

enough so that the unit will notinterfere with other TV viewerswho are nearby. To complywith FCC rules, it is mandatorythe nearby TV viewers are notdisturbed by the transmission. Ifyour activities interfere with thereception from a licensedstation, regardless of the reason,you must shut down your unit.

Circuit DescriptionFigure 1 is the schematic

diagram of the TV Transmittercircuit. Video signals input atjack J1 are first terminated byresistor R6 and coupled throughcapacitor C1 to clamping-diodeD1. The clamping forces thesync pulses to a fixed DC levelto reduce blooming effects.Potentiometer R3 is used to setthe gain of the video signal; itseffect is similar to that of thecontrast control on a TV set.Bias-control R7 can be used toadjust the black level of thepicture so that some level ofsignal is transmitted, even for atotally dark picture. That way, aTV receiver can maintain proper

sync. As we'll get to later, potentiometersR3 and R7 are cross adjusted for the bestall-around performance.

RF-transformer T1 and its internalcapacitor form the tank circuit of a Hartleyoscillator that's tuned to 4.5 megahertz.Audio signals input at J2 are coupled to thebase of Q3 via C2 and R4: the audio signalmodulates the base signal of Q3 to form anaudio subcarrier that‚s 4.5-megahertzhigher than the video-carrier frequency.The FM modulated subcarrier is applied tothe modulator section through C5 and R9.Resistor R9 adjusts the level of thesubcarrier with respect to the video signal.

Transistors Q1 and Q2 amplitudemodulate the video and audio signals ontoan RF-carrier signal. The operatingfrequency is set by coil L4, which is 3.5turns of 24- gauge enameled wire on aform containing a standard ferrite slug.

Fig. 1. The schematicshows how easy it isto hook up the TVTransmitter to otherequipment. There isa video-input jack,J1; an audio-inputjack, J2; and anexternal- antennajack, J3. You couldeliminate the latterbecause the unitworks fine with justan installedtelescoping antenna.

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That coil is part of a Colpitts tank circuitalso containing C7 and C9. The tank circuitforms Q4's feedback network, so Q4oscillates at the set frequency

The RF output from the oscillatorsection is amplified by Q5 and Q6, whosesupply voltage comes from the modulatorsection. Antenna matching and low-passfiltering is performed by C12, C13, and L1.Resistor R12 is optional; it is added to helpmatch the output signal to any kind ofantenna. (More on that in a moment.)

ConstructionBefore we go on, while it is certainly

possible to build the unit from scratch.However, unless you are an experiencedbuilder and an accomplished partsscrounger, it is strongly recommended thatyou purchase the complete kit, or, at thevery least, the component kit from thesource mentioned in the Parts List. Whilemost of the parts are readily available,some can be a real headache to obtain.

The 4.5-MHz RF transformer (T1) usedin the kit is an OEM Toko part that is notavailable via traditional sources. While justabout any 4.5-MHz RF transformer that issimilar to the one described in the article(internal capacitor, tapped secondary) canbe used, such units are hard to obtain fromhobbyist-friendly sources. If you aredetermined to go that route, your best betis to contact Toko directly (1250Feehanville Dr.. Mt, Prospect, IL 60056;Tel. 708-297-0070) to obtain the location ofyour nearest full-line distributor. Also, coilL4 is a custom unit. It can, however, behome made using the parameters givenearlier.

The Transmitter should be built on aPC board for best performance. You canmake a board from the foil patternprovided in Fig. 2, or use the one that’sincluded with the kit.

Parts are installed on the board asshown in the parts-placement diagram[see Fig. 3). Pay careful attention to theorientation of the transistors, electrolyticcapacitors. and the diode. If resistor R12(not included in the kit) is used, it must betack- soldered on the solder side of the

boardbetween theantennaoutput andground.That resistorshould beinstalled ifyou intendto useanythingother thanthe built- in whip to provide propermatching between the antenna and thecircuit.

The outline of the switch (S1) that isshown in Fig. 3 is the same as the one thatcomes with the kit, an SPST push-buttonswitch that is normally open. You can useany kind of toggle switch as a replacement.A simple whip antenna mounts to theboard with a single machine screw: The

This case allows easyaccess to the circuit boardfor alignment purposes.

4 INCHES

Fig. 2. If you wish,you can use thisfoil pattern diagramto make your ownPC board for theTV Transmitter.

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whip antenna is suitable for mostapplications. The battery holder can besoldered to the board with scraps ofjumper wire or mounted with double-sided tape or screws.

When the board is finished, it must bemounted in a case. The case available fromRamsey Electronics allows the board to bemounted in the bottom half, and by liftingthe top off, still be aligned. That alsoprotects the underside of the board againstshorts during alignment. You shouldinspect the solder side of the boardcarefully before mounting it in the case.

AlignmentTo align the TV Transmitter, you'll

need a TV receiver and a source of videosuch as a VCR or camcorder. You'll alsoneed a non-metallic tool to adjust coil L4and transformer T1. A fresh 9-volt batterycan be used for alignment, but if you findit is difficult to align, try doing it with a 12-

volt supply. Note that during alignmentand testing, we found that the unitoperated much better from 12 volts. If youfind the same to be true, it is a simplematter to add an external power jack to the

unit and wire it to the appropriatepoints on the PC board.

Tune a TV receiver to an unusedchannel between 2 and 6. The TVmust have an indoor antennaconnected directly to it; an outdoorantenna or cable won't work. Makesure both potentiometers are in mid-position and apply power to theTransmitter. Adjust L4 with anonmetallic tool until the TV screengoes blank. Then fine-adjust L4 forthe "most-blank" picture.

Connect the video and audiooutputs from a VCR to jacks J1 and J2(respectively) of the Transmitter,Then set a video tape to play. Youshould see a picture on the TV screen:if you do, readjust L4 for the bestpicture; if you don't, check the boardfor any bad connections. Next, adjustR3 for the best picture brightness andR7 for the best overall picture. Youmight have to make another minoradjustment to L4 after R3 and R7 areset. Finally, adjust T1 with a non-metallic tool for the best soundingaudio. That‚s all there is to it.

The whip antenna should be finefor most in-home use. If you need morerange, an external antenna can beconnected to J3 (remember to install R12).But always keep in mind that it is yourresponsibility to make sure that youroperation does not interfere with yourneighbor's TV viewing.

Fig. 3. Buildingthe project is easyif you use thisparts-placementdiagram. ResistorR12 must be tack-soldered on thesolder side of theboard between theantenna outputand ground.

S1

B1 +

Q4

Q6 Q5Q1Q2

Q3 R12*

J3*SEE TEXT

J1 J2

C3

R5

R2

C4

R4C2

D1

C1+

R6

R7 R3C5

R9

T1 J

C11

C9

R11C12 C13

C14L1

ANT1

L4

C7

C10R10

L2

L3

C8

C15

R1

C6

R8

+

+

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Copyright © 1997, POPtronix Inc.All rights reserved.As a service to readers, POPtronix, Inc. publishes available plans or information relating to newsworthy products, techniquesand scientific and technological developments. Because of possible variances in the quality and condition of materials andworkmanship used by readers, we disclaim any responsibility for the safe and proper functioning of reader-built projectsbased upon or from plans or information published online.

Since some of the equipment and circuitry described in POPtronix, Inc. Project Articles may relate to or be covered by U.S.patents, we disclaim any liability for the infringement of such patents by the making, using, or selling of any such equipmentor circuitry, and suggest that anyone interested in such projects consult a patent attorney.

PARTS LIST FOR THETV TRANSMITTER

SEMICONDUCTORSD1—1N914 silicon diodeQ1-Q—2N3904 NPN transistor

RESISTORS(All fixed resistors are 1/4-watt, 5% units .)R1, R2, R11—1000-ohmR3, R7—1000-ohm trimmer potentiometer, PC-mountR4, R9, R10—10,000-ohmR5—47,000-ohmR6—75-ohmR8—4700-ohmR12—75-ohm (optional, see text)

CAPACITORSC1, C8—100-µF, 16-WVDC, electrolyticC2—2.2--µF, 50-WVDC, electrolyticC3-C6, C11, C14, C15—001-µF, ceramic-discC7, C9—2.2-pF, ceramic-discC10—100-pF, ceramic-discC12, C13—68-pF, ceramic-disc

ADDITIONAL PARTS ANDMATERIALSANT1—Antenna, telescopic-whipB1—9-volt batteryJ1-J3—RCA jack, PC-mountL1—0.15-µH miniature inductorL2, L3—2.2-µH miniature inductorL4—0.14- to 0.24-mH adjustable, slug-tuned coil(see text)S1—SPST, push-button switch, normally openT1—4.5-MHz 1F-can-style RF transformer (seetext)Printed-circuit materials or pre-fab PC board,

battery holder and connector, pair of RCApatch cords, solder, hardware, etc.

Note: The following items are available from Ramsey Electronics, Inc. 793 Canning Parkway Victor, NY 14564 Tel. 716-924-4560TV-6 TV Transmitter Kit (includes PC board and allcomponents except R12)—$27.95; kit of allcomponents (except R12)—$17.95; PC boardonly—$10.00; CTV matching-case set—$14.95.NY State residents please add appropriate salestax.

WARNING!!The publisher makes no representationsas to the legality of constructing and/orusing the TV Transmitter that isreferenced in this article. Theconstruction and/or use of thetransmitter described in this article mayviolate federal and or state law. Readersare advised to obtain independentadvice as to the propriety of itsconstruction and the use thereof basedupon their individual circumstances andjurisdiction.