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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| Click here for circuit dia.
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