An intrusion detection system employs a microwave subsystem and a passive infra-red subsystem. Both systems must produce an output signal indicative of an intrusion in order for the system to produce an alarm. There is disclosed a supervision circuit which monitors the number of trips of the microwave...http://www.google.co.uk/patents/US4833450?utm_source=gb-gplus-sharePatent US4833450 - Fault detection in combination intrusion detection systems
