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 transistors
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 listTransistorQ1, Q2 : BC547Q3 : BF494DiodeD1D4 : 1N4007D5
: 18V Zener-diodeResistorsR1 : 100KR2, R6 : 22KR3, R8 : 47KR4 :
470KR5 : 33KR7 : 150KR9 : 470RCapacitorsC1, C3 : 1063V,
electrolyticC2, C4 : 100n or 100Kpf, ceramic-discC5 : 1n or 1Kpf,
ceramic-discC6 : 33p, ceramic-discC7, C8 : 10p,
ceramic-discAdditional partsL1- 4Turns, 22SWG Copper wire, 3mm air
coreVC- 2-22pf, Trimmer CapacitorANT- 10 to 80cm copper wirecommon
PCB
