Centralised security surveillance system - has logic circuit initiating alarm if abnormal condition persists for specified time period

摘要

The centralised security surveillance system, for buildings etc., has signals received from respective detectors, with normally closed contacts, applied to a series of input paths connected via a coupling circuit to the alarm circuit. The coupling circuit includes a logic circuit, connected to a monostable continuity detection circuit, providing an output pulse with a given delay after reception of an abnormal signal from one of the input paths. A bistable detection circuit has its clock input coupled to the output of the monostable circuit and its control input coupled to the input of the latter, for operating the alarm if the abnormal signal persists without interruption through out the latter delay period.