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
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