FM telephone bug.


A simple telephone transmitter circuit that is ideal for transmitting the telephone conversation through small distances.The circuit is very simple and uses only few components.The entire circuit can be easily included in the telephone itself or in the junction box.The circuit is powered from the telephone line itself.

The transistor Q2 is wired as a a Colpitts oscillator to produce oscillations in the FM transmission band.The audio signal from the telephone line is coupled to the the base of Q2 to obtain the frequency modulation.The LED D1 acts as a power on indicator.

Circuit diagram with Parts list.



  • Any radio frequency PNP transistor like A933 can be used for Q2.
  • For L1 make 8 turns of 0 .6mm dia enameled copper wire on a 3.6mm dia steel bolt.
  • For antenna use a 15cm long 0.6 mm dia copper wire.