Monitoring circuit for an ancillary telephone device

摘要

To check the voltage of a telephone line, a capacitor is connected for a fixed period via a high-impedance resistor between the two telephone wires by a monitoring circuit of an ancillary telephone device and is then discharged via a low-impedance resistor and two back-to-back optocouplers. Depending on the result of the check, corresponding signals are transmitted by the ancillary telephone device for evaluation by danger alarm systems.