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Phone
Broadcaster
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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.
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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. |
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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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