Detecting continuous current in two wire telephone line - is performed to sense open or closed loop circuits

摘要

The various loop circuits in telephone network use continuous current. Open or closure of loops is sensed by detecting current flow to signal lifting or replacement of handset or dialling process. Current (I) flowing in two wire connexion circuit passes through resistance (11) and causes transistor (12) to saturate or be non conducting. A diode (17) between transistor base and one conductor (18) protects transistor and photo-coupler (15) diode (14) in its collector circuit. A resistor (13) allows photodiode current to be adjusted for long signalling life. A resistor (r) in other line provides network symmetry.