| 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. |
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