Schematics > Telephone related > Multipurpose Circuit for telephone
Multipurpose Circuit for telephone
This add-on device for telephones can be connected in parallel to the
telephone instrument.
The circuit provides audio-visual indication of on-hook, off-hook, and ringing
modes. It can also be used to connect the telephone to a cid (caller
identification device) through a relay and also to indicate tapping or misuse of
telephone lines by sounding a buzzer.
In on-hook mode, 48V dc supply is maintained across the telephone lines. In
this case, the bi-colour led glows in green, indicating the idle state of the
telephone. The value of resistor r1 can be changed somewhat to adjust the led
glow, without loading the telephone lines (by trial and error).
In on-hook mode of the hand-set, potentiometer vr1 is so adjusted that base
of t1 (bc547) is forward biased, which, in turn, cuts off transistor t2 (bc108).
While adjusting potmeter vr1, ensure that the led glows only in green and not in
red.
When the hand-set is lifted, the voltage drops to around 12V dc. When this
happens, the voltage across transistor t1’s base-emitter junction falls below
its conduction level to cut it off. As a result transistor pair t2-t3 starts
oscillating and the piezo-buzzer starts beeping (with switch s1 in on position).
At the same time, the bi-colour led glows in red.
In ringing mode, the bi-colour led flashes in green in synchronisation with
the telephone ring.
A cid can be connected using a relay. The relay driver transistor can be
connected via point a as shown in the circuit. To use the circuit for warning
against misuse, switch s1 can be left in on position to activate the piezo-buzzer
when anyone tries to tap the telephone line. (When the telephone line is tapped,
it’s like the off-hook mode of the telephone hand-set.)
Two 1.5V pencil cells can provide Vcc1 power supply, while a separate power
supply for Vcc2 is recommended to avoid draining the battery. However, a single
6-volt supply source can be used in conjunction with a 3.3V zener diode to cater
to both Vcc2 and Vcc1 supplies
Title: Multipurpose Circuit for telephone
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-09
Reads: 394
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