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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
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