Alarm system.

摘要

An alarm system includes at least one detecting device (10) which provides an analogue output signal (12), means (18) to monitor the rate of change of the analogue output signal and provide an alarm signal (50) when the rate of change exceeds a preset limit, and means (20) to compare the analogue output signal to a first reference level (20A) and provide a pre-maintenance warning signal when the analogue output signal reaches the first reference level.

The rate monitoring means (18) comprises a sample-and-hold circuit (40), a difference amplifier (42) and a comparator (50). Circuit (40) holds the signal (12) for a predetermined time and amplifier (42) substracts a predetermined small voltage (46). If in the holding time the signal (12) changes by more than voltage (46), comparator (44) produces signal (50) indicative of the rate of change exceeding the preset limit. Drift of signal (12) to a level (20A) indicates the need for maintenance and reaching upper limit (22A) or lower limit (24A) indicates that detector (10) is inoperative.