Here
is a simple yet very useful circuit which can be used
to eavesdrop on a telephone conversation. The circuit
can also be used as a wireless telephone amplifier.
One important feature of this circuit is that the
circuit derives its power directly from the active telephone
lines, and thus avoids use of any external battery or
other power supplies. This not only saves a lot of space
but also money. It consumes very low current from telephone
lines without disturbing its performance. The circuit
is very tiny and can be built using a single-IC type
veroboard that can be easily fitted inside a telephone
connection box of 3.75 cm x 5 cm.
The circuit consists
of two sections, namely, automatic switching section
and FM transmitter section.
Automatic switching
section comprises resistors R1 to R3, preset VR1, transistors
T1 and T2, zener D2, and diode D1. Resistor R1, along
with preset VR1, works as a voltage divider. When voltage
across the telephone lines is 48V DC, the voltage available
at wiper of preset VR1 ranges from 0 to 32V (adjustable).
The switching voltage of the circuit depends on zener
breakdown voltage (here 24V) and switching voltage of
the transistor T1 (0.7V). Thus, if we adjust preset
VR1 to get over 24.7 volts, it will cause the zener
to breakdown and transistor T1 to conduct. As a result
collector of transistor T1 will get pulled towards negative
supply, to cut off transistor T2. At this stage, if
you lift the handset of the telephone, the line voltage
drops to about 11V and transistor T1 is cut off. As
a result, transistor T2 gets forward biased through
resistor R2, to provide a DC path for transistor T3
used in the following FM transmitter section.
The
low-power FM transmitter section comprises oscillator
transistor T3, coil L1, and a few other components.
Transistor T3 works as a common-emitter RF oscillator,
with transistor T2 serving as an electronic ‘on’/‘off’
switch. The audio signal available across the telephone
lines automatically modulates oscillator frequency via
transistor T2 along with its series biasing resistor
R3. The modulated RF signal is fed to the antenna. The
telephone conversation can be heard on an FM receiver
remotely when it is tuned to FM transmitter frequency.
Lab Note: During testing of the circuit it was observed
that the telephone used was giving an engaged tone
when dialed by any subscriber. Addition of resistor
R5 and capacitor C6 was found necessary for rectification
of the fault. |
