Telephone Watchdog

This simple circuit will tell you, by sounding a buzzer and flashing a LED, that a telephone line is in use. If the telephone line is free, it will carry a steady direct voltage of something between 48 and 60 volts. This is well above the zener voltage of D5 so optocoupler IC1 is switched on and consequently the oscillator built around IC2 is held disabled.When the line is in use, i.e., a telephone extension is ‘off hook’, the line voltage drops to a level below the zener voltage of D5, and the oscillator is enabled. LED D6 will flash and buzzer Bz1 will produce intermittent beeps.