Abstract: A system for detecting intrusion into a protected area by virtue of a change in detected infrared energy from an ambient level, and for generating an alarm signal in response thereto includes a first assembly having a Passive Infra Red (PIR) sensing element for generating a contrast signal representative of deviation in detected infrared energy, a second assembly for generating an ambient temperature signal, an amplifier for amplifying the contrast signal, and a processor for generating a threshold as a function of the ambient temperature. The gain and threshold are defined to generate an "alarm trigger condition", and an alarm activator responds to the "alarm trigger condition" for activating an alarm signal.