FM Telephone Bug

Here is a simple transmitter that when connected to a phone line, will transmit anything on that line (execpt the dial tone) to any FM radio. The frequency can be tuned from 88 to about 94Mhz and the range is about 200 feet. It is extremely easy to build and is therefore a good, useful beginner project.


This is the schematic of the phone transmitter


Part Total Qty. Description Substitutions
R1 1 180 Ohm 1/4 W Resistor  
R2 1 12K 1/4 W Resistor  
C1 1 330pF Capacitor  
C2 1 12pF Capacitor  
C3 1 471pF Capacitor  
C4 1 22pF Capacitor  
Q1 1 2SA933 Transistor  
D1, D2, D3, D4 4 1SS119 Silicon Diode  
D5 1 Red LED  
S1 1 SPDT Switch  
L1 1 Tuning Coil  
MISC 1 Wire, Circuit Board  


1. L1 is 7 turns of 22 AWG wire wound on a 9/64 drill bit. You may need to experiment with the number of turns.

2. By stretching and compressing the coils of L1, you can change the frequency of the transmitter. The min frequency is about 88 Mhz, while the max frequency is around 94 Mhz.

3. The green wire from the phone line goes to IN1. The red wire from the phone line goes to IN2. The green wire from OUT1 goes to the phone(s), as well as the red wire from OUT2.

4. The antenna is a piece of thin (22 AWG) wire about 5 inches long.

5. All capacitors are rated for 250V or greater.

6. The transmitter is powered by the phone line and is on only when the phone is in use. S1 can be used to turn the transmitter off if it is not needed.

7. If you have problems with the LED burning out, then add a 300 ohm 1/4W resistor in series with it.

Title: FM Telephone Bug
electronic circuit
Published on: 2005-02-10
Reads: 1295
Print version: Print version

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