Passive infrared detector adopts passive infrared method, which has achieved security alarm function. Passive infrared detectors are mainly composed of optical 

system, pyroelectric sensor (or infrared sensor) and alarm controller. The detector itself does not emit any energy but passively receives and detects infrared radiation from the environment. Once the infrared radiation of the human body comes in, the pyroelectric device will generate a sudden change of electric signal by focusing by the optical system, which will sound an alarm.