Telephone line status detecting unit - has photocoupling device to detect whether line current is lower or greater than set value

摘要

A telephone-line status-detecting unit can determine when the line current is lower or greater than set values. It can also be insensitive to the line cut-off due to dialing a number. The unit has a photo-emitting diode circuit (1) connected to a telephone line (L). It also has a photo-coupling device (5) and a photo-detecting diode (6). The photo-emitting diode (4) is connected to a resistor bridge (32, 33) and to the collector of a transistor (30). The transistor (30) is also connected across the line (L). The photo-detecting diode (6) is connected to a circuit (7) with an assymetrical time constant. This circuit (7) has a network with a diode (37), a resistor (38) and a capacitor (400). The circuit (7) has a flip-flop action. The logic circuit (7) is connected to a locking circuit (9) with an inhibition input terminal (15) and a sampling input terminal (16).