INFRA-RED INTRUDER DETECTOR

An infra-red intruder detection system has two separate channels A and B each having its own pyroelectric detector device (10A, 10B) and threshold level comparators (22A, 23A, 22B, 23B) responsive to dual polarity pyroelectric voltage outputs produced by the associated detector device as a result of an intruder-related infra-red image moving thereacross. A logic circuit (25) analyses the comparator outputs and responds to the identification of particular intruder-indicative sequences of comparator outputs in both channels to generate an alarm and/or switching output (27). The logic circuit may also identify shock-induced comparator outputs. The system offers a high intruder detection capability with immunity from false triggering.