Rectifying the buzzer signal on a ” telephone line results in a voltage that may be used to switch one or more loads, for instance, a light to show the deaf or hard-of-hearing that the telephone is ringing.
In the present circuit, the buzzer signal is rectified by D1-D4; the rectified voltage is smoothed by C2 and held at 15 V by zener diode D5. The voltage is used to drive the LED in optoisolator IC1 via R3. The optoisolator provides safe isolation between the telephone line and the present circuit and its load. Note, however, that in spite in this, the telephone authorities in some countries may not permit the use of the present circuit: it is always best to seek the advice of your local telephone manager.
As soon as a call signal appears on the telephone line, the LED in the optoisolator ensures that the integral transistor is switched on. This in turn switches on whereupon relay Re1 is energized.
Capacitor C3 is charged as long as the transistor in the optoisolator conducts, but discharges via R4-R5-T1 when the call signal fails: this prevents T1 being switched off during the intervals between the various pulses of the call signal. When that signal fails, the relay will be denergized after a short delay.
The circuit can be supplied by a 12 V mains adapter. The current it draws depends on the type of relay used, but should not exceed 100 mA. Make sure that the relay can handle the switched voltages and currents.