FM Telephone Transmitter, Bug

FM Telephone Transmitter

    The complete schematic of the telephone bug is shown in Fig.1. This circuit connects in parallel with the telephone line. Power for the circuit is full wave bridge rectified from the phone line by diodes D1 through D4. When the telephone receiver is on hook the voltage across the lines will be about 48 volts. At this voltage the Zener diode D5 will be in breakdown and the transistor Q1 will conduct. This makes the transistor Q2 OFF. When the receiver is off hook, the line voltage drops to about 12 volts. This makes the transistor Q1 OFF and subsequently the Q2 ON. The Q2 in switched ON condition will provide a DC path for the transistor Q3 in the FM transmitter section. The transmitter section is actually a small FM transmitter that can be turned to a vacant spot on your Fm radio. In the FM frequen­cy range, only a small antenna is needed to radiate a usable signal through out a building. Its range is 50 to 100 feet with a 10cm antenna. The range can be increased by using a longer antenna. Transistor Q3 oscillate at a signal frequency which is determined by turned circuit L1, C6, VC and the inter electrode capacitance of the transistor. Capacitor C7 provides the need­ed positive feed back from the collector to the emitter for the circuit to oscillate. The audio signals applied to the base of Q3 cause collector current to vary at an audio rate and modulate the oscillator output. This type of modulation produces both FM and AM, but AM are rejected by the discriminator and limiters in the FM receiver and cause no problem. The collector current variations in turn vary the transistor’s output capacitance. This capacitance is part of the turned circuit and varies the frequen­cy of the output signal which is coupled to the antenna by C9.

FM Telephone Transmitter Circuit Diagram

FM Telephone Transmitter PCB

FM Telephone Transmitter PCB Component Side

 

Parts list

Transistor

Q1, Q2               : BC547

Q3                      : BF494

Diode

D1…D4              : 1N4007

D5                      : 18V Zener-diode

Resistors

R1                      : 100K

R2, R6                : 22K

R3, R8                : 47K

R4                      : 470K

R5                      : 33K

R7                      : 150K

R9                      : 470R

Capacitors

C1, C3                : 10µ63V, electrolytic

C2, C4                : 100n or 100Kpf, ceramic-disc

C5                      : 1n or 1Kpf, ceramic-disc

C6                      : 33p, ceramic-disc

C7, C8                : 10p, ceramic-disc

Additional parts

L1- 4Turns, 22SWG Copper wire, 3mm air core

VC- 2-22pf, Trimmer Capacitor

ANT- 10 to 80cm copper wire

common PCB