ON-HOOK/OFF-HOOK DETECTOR CIRCUIT

摘要

An on-hook/off-hook detector circuit (10) is included within a telecommunication integrated circuit. A tone generator power signal (Vu) is monitored as a status signal to determined when a telephone switch hook is in either an on-hook or off-hook condition. The tone generator power signal is subject to interference and transients which prevents direct utilization of it to indicate on-hook and off-hook conditions. A resistor (20) and capacitor (22) are connected externally to the circuit (10) at a junction terminal (16) to provide a time delay following a transition of the status signal (Vu). The junction terminal of the resistor (20) and capacitor (22) is held at two transistor threshold voltages below the upper supply voltage (V +). At the time of transitions of the status signal CVu the junction terminal (16) is disconnected from a current path such that the terminal (16) is permitted to charge the capacitor (22) through the resistor (20). When the capacitor (22) has charged up to within one threshold voltage of the positive power terminal (V +) a transistor (48) is turned off thereby permitting the on-hook signal at line (50) to be driven negative thereby reliably indicating that the switch hook has been placed in the on-hook condition. The charging of the capacitor (22) serves as a time delay to eliminate faulty signal indications derived from noise and transients associated with the status signal (Vu).