Multipurpose Circuit For Telephones

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 re-lay 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 some-what to adjust the  LED glow, with-out 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, en-sure that the  LED glows only in green and not in red.

Multipurpose Circuit For Telephones Circuit Diagram

When the handset is lifted, the volt-age 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 synchronization 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 con-junction with a 3.3V zener diode to cater to both Vcc2 and Vcc1 supplies.