CALL SIGNAL TRANSMISSION CIRCUIT

PURPOSE:To reduce a mounting space, to attain cost-down and to prevent a spark noise by providing a photocoupler having a DC high voltage generating source and a photodetection section, a low frequency pulse oscillator and a photocoupler drive circuit applying its output pulse intermittently to a light emitting section of the photocoupler at a prescribed so as to miniaturize the oscillator. CONSTITUTION:When a master device 1 detects an incoming call from a trunk line or an extension to a key telephone set 2, the device 1 transmits an incoming detection signal to a call signal start circuit 10. When the circuit 10 receives the incoming detection signal, a start control signal (p) as the repetitive signal of 1 sec high level H and 2 sec low level L is outputted. On the other hand, an oscillator 8 generates normally a 16Hz rectangular wave. Since the start control signal (p) is at a low level L until the incoming detection signal is received, photocouplers PHC1-PHC3 are in the off-state during this time and no DC high voltage is fed to an incoming tone generating circuit 4. When the incoming detection signal is received and the start control signal (p) is at a high level H, the output of an inverter INV6 goes to a low level L and the photocoupler PHC3 is turned on.