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. Circuit diagram: Multipurpose Circuit For Telephones circuit Diagram When the handset i...