Phone Broadcaster
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.
Title: Phone Broadcaster
electronic circuit
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-09
Reads: 691
Print version:
Other electronic circuits and schematics from Telephone related
-
Telephone amplifier
-
Cordless phone backup
-
Telephone call meter using calculator & COB
-
Phone line indicator
-
Telephone Hold Button
-
Soft Musical Telephone Ringer
-
The Original 2 Phone Link Design
-
Telephone Headgear
-
FM Telephone Bug
-
The Link A2B+1 (the Link Telephone Intercom - DTMF version)