This add-on devices 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, with-out loading the telephone line (by trial and error).
In In on-hook mode of the handset, potentiometer VR1 is so adjusted that base of T1 BC547 is forward biased, which, in turn, cuts off transistor T2 BC108. While adjusting potentiometer VR1, ensure that the LED glows only in and not in red.
When the handset 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 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 diver 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 peizo-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 handset.)
Two 1.5 volt 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 3.3 volt zener diode to cater to both Vcc2 and Vcc1 supplies.
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