A very simple electronic phone fm transmitter can be constructed using this circuit diagram . This FM transmitter circuit can be attached in series to one of your phone lines to transmit the received signal on the FM band . When you pick up the phone , signal is present on the line the circuit will transmit the conversation on a short distance. This phone fm transmitter circuit will not need any power supply or battery , because this circuit use the signal from the phone line for power .
No aerial is needed - it feeds back the RF signal into the phone line which radiates it in the FM band. The frequency of transmission can be adjusted by the trimcap.
To set the desired operating frequency of this phone fm transmitter circuit you can take your phone off the hook and turn on an FM radio at about 93 MHz and tune the circuit into transmission . After that , take a portable FM receiver and search the signal from the phone line.
Usually this type of circuit should be calibrated , but it works fine without any calibration .
The T1 transistor can be BC547 or BC548 and the T2 transistor can be ZTX320 or similar type .
All diodes used in this fm transmitter project are 1N4148 type and the C8 capacitor is a 5-20pF trimcap (variable capacitor ) .
>For this transmitter you need to construct the inductors so : L1 has 6 turns enamelled Cu wire , L2 has 8 turns enamelled Cu wire and L3 has 6 turn tinned Cu wire .