Tracking Transmitter

Circuit : Andy Collinson
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This is a small battery operated Tracking transmitter which broadcasts a repeating radio signal which can be detected by a directional antenna. The transmitter broadcasts an isotropic (radiates in all directions) signal on the FM band. An ordinary FM radio can be used to locate the transmitter, the reception of the signal will improve when the radio antenna is pointed in the direction of the transmitter.

tracking transmitter circuit Notes
The transmitter is a standard hartley oscillator, designed to transmit across the FM band approximately, 87 to 108MHz. The transmit frequency is adjustable by means of trimmer C8. The combined capacitance of C4 plus C8 and L1 set the resonant frequency.

Modulation for the transmitter is an interrupted square wave signal using all 4 gates of a CD 4011. Gates U1 and U2 interrupt the signal at about 2Hz while gates U3 and U4 create a higher tone in the audio range. The output of U4 applies this modulation to the base of Q1 via R3. Because the modulation is a squarewave, the radiated signal becomes a little bit "dirty" and harmonics will be heard across the FM band. This is not a big problem as it will aid in locating the transmitter.

Tuning the Transmitter
The transmitter is tuned by the tank circuit L1 and parallel capacitance of C4 plus C8. L1 is approximately 0.15 uH. The value of "C" in the equation below is the sum of C4 plus C8.

Resonant Frequency Calculator
Resonant Frequency:

Construction of L1
L1 is 7 turns of 20swg wire wound on a 6 mm drill bit. This forms an inductor of approximately 150 nH. The value of the inductance can be altered by compressing or expanding the turns.

Start with a 5 inch length of 20swg enamel coppered wire. Scrape about 3mm of the insulation from one end with a penknife or sandpaper.

Hold one end of the wire against the drill bit and start to wind the turns clockwise. Continue until all 7 turns have been completed, and the coil looks like the image of the right hand side. Trim any excess wire so both ends are same length and scrape about 3mm of insulation off the opposite end.

If preferred commercial coils may be used. Some have moulded cores and adjustable ferrite slugs to alter the inductance, as shown (right).
Inductance Calculator
For anyone wishing to wind their own coils, Ron J kindly submitted a javascript calculator, on my tuned circuit page. It is presented here again for convenience. For example using a diameter of 6mm (0.6cm) and 1cm length, a desired inductor of 0.15uH works out at 7.33 turns.
† Diameter :
Length :
Inductance (µH): Turns
† External Diameter
Test Transmitter
I made the circuit on breadboard, shown below. It performed quite well on a 9Volt power source. Current drain is about 35mA but with just 4 inch of wire as antenna the signal could be received easily outside my home about 20 metres away.

project on veroboard

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