The FM Transmitter Bug

Building an FM transmitter is fun! This kit was very easy to build so it would be a good first time kit if you’re just getting started in electronics. If you can understand how to read capacitor and resistor values and you can solder, you can build this kit. If you’d like to build one they can be purchased from these vendors:
http://www.hobbyengineering.com/H1481.html
http://www.hobbytron.com/Mini-FM-transmitter-3-Volt-FM-Transmitter-bug-kit.html.

My Friend John S. gave this kit to me recently and I thought it would be a good Hack A Week project. There’s some good electronics to learn about here: class C amplifiers, FM transmission, VHF antennas, positive and negative feedback, stray capacitance, and induction.

The circuit is basically an audio amplifier that modulates a radio frequency oscillator tuned around 100 MHz. Output from the collector of the amplifier transistor is fed into the base of the second transistor where it modulates the resonant frequency of the “tank” circuit (the 5 turn coil and trim capacitor) by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is connected to a Hartley oscillator circuit.

I was able to receive transmissions out to 30 meters and that was going through a metal garage door. The range can be greatly extended by using a dipole antenna rather than the half wave antenna. Connect the dipole to the circuit with a 50 ohm coaxial cable. Connect the center lead to the antenna point on the circuit and the outer shield to earth ground. I’ll be experimenting more with this.

Have fun building this and keep on hackin!

Component list:

Resistors: (carbon, 0.25 W, 5%)
1 – 1Megohm
1 – 47K ohm
1 – 22K ohm
1 – 10K ohm
1 – 470 ohm

Capacitors:
1 – 1n ceramic #102
1 – 5pf ceramic
2 – 22n #223
1 – 27pf ceramic
1- 100n monoblock

Other stuff:
2 – BC547 transistors
1 – 5 turn coil (make sure you separate the windings so they don’t touch each other)
1- Electret microphone
1 – tripcap, yellow (6 – 45 pf)
165 cm antenna wire
Battery holder, 3v – 9v.

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