| 摘要 |
PURPOSE:To detect the answering state of a called telephone and stop the transmission of the call signal, by providing an output holding circuit to the output of a current holding circuit and controlling an operating circuit of a call signal sending relay through the output holding circuit. CONSTITUTION:A control signal SD is applied when a call signal R is transmitted to a called telephone. A relay RL works by the signal SD, and the signal R is sent to a called telephone TEL. When the telephone TEL gives an answer, a DC voltage -V is applied among the terminals of a capacitor C. The voltage -V exceeds the breakdown voltage of a constant voltage diode D, and thus the pulsating current flows via a resistance R4 to actuate a photocoupler PH. With actuation of the PH, an input logic level varies to a gate G2. As a result, a gate G3 is kept under conduction, and no signal SD is supplied to a driver DV to ensure a quick release of the relay RL. |
