Abstract: Embodiments of a passive infrared detector can distinguish between an intruder and a small animal passing nearby the detector with a high degree of accuracy without substantially detracting from the ability to detect the intruder within a coverage area farther from the detector. In some embodiments, the passive infrared detector includes a pair of lens assemblies that collect infrared light onto corresponding infrared sensors. Each lens assembly receives light from different spatial regions, which do not overlap near the detector and which substantially overlap farther from the detector. The passive infrared detector also includes a controller that is capable of distinguishing between an intruder and a small animal based on simultaneity of signals from the infrared sensors or by summing the signals from the infrared sensors.

Abstract: The present invention aims to avoid mutual interference of outputs of passive infrared detectors in an amplifier circuit of an infrared sensor to be amplified by a single system of amplifier circuit by cyclically switching outputs from a plurality of passive infrared detectors at a predetermined cycle. P.sub.1 and P.sub.2 each represents a passive infrared detector, and switches 2.sub.1 and 2.sub.2 can be switched over cyclically. Between an operational amplifier 12 and an operational amplifier 13, series circuits of capacitors and switches are provided in parallel as many as the passive infrared detectors. Switches 20.sub.1 and 20.sub.2 are opened and closed in synchronization with the switches 2.sub.1 and 2.sub.2 respectively. The capacitor C.sub.30 is used when an output from the passive infrared detector P.sub.1 is amplified, and a capacitor C.sub.31 is used when an output from the passive infrared detector P.sub.2 is amplified.