Circuit : Andy Collinson
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This is a small low power two transistor Colpitts type oscillator. Centered on 101MHz FM it can be used as a small room bug. Range is 15 to 20 meters with a few centimetres of wire.
NotesI have used a BC108B as an audio amplifier and a MPSH11 transistor for the RF oscillator stage. MPSH10 can also be used and is available at Maplin and Bowood electronics. I have used an ECM Mic insert from Maplin Electronics, order code FS43W. It is a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit R1.
The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. The two 82pF capacitors in series have a capacitance of 41pF. This gives a resonant frequency of about 101.7MHz. Some adjustment is possible by squeezing or expanding the coil. If you prefer a tunable transmitter than the other transmitters are better suited to your needs.
Although RF circuits are best constructed on a PCB, you can get away with veroboard, as long as you are careful with the design. The coil and C4 and C5 need to be near each other and any long tracks need to be cut, otherwise they may add parasitic capacitance.
Coil Details
Wind 5 turns of 0.5mm diameter copper wire over a 5mm drill bit. Turns should be spaced so that overall length is about 1cm. This creates an inductance of approximately 60nH.
A turns calculator is available on my tuned circuit page.
One final point, don't hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations. I have had some first hand experience of this problem. The frequency of oscillation can be found from the theory section,and an example now appears in the Circuit Analysis section.