There are two key components in a passive infrared detector. One is the Fresnel lens, and the other is the pyroelectric sensor. Any object in nature with a temperature above absolute zero (-273℃) will emit infrared radiation. The wavelength of the infrared energy released by objects of different temperatures is also different. The human body has a constant temperature, which is different from the surrounding environment. When the human body moves, the change in this difference is detected by the pyroelectric sensor through the Fresnel lens, and an alarm signal is then output. 

Passive infrared detectors do not require an additional infrared radiation source. They do not emit any energy to the outside world. Instead, they directly detect the infrared radiation from moving targets, which is why they are called "passive". 

The core component of the passive infrared detector is the infrared detection device (infrared sensor). Through the combined action of the optical system, it can detect the changes in thermal radiation within a certain three-dimensional security space. When there is no moving human or other targets within the security area, since all background objects (such as walls, furniture, etc.) have relatively small infrared radiation energy at room temperature and are basically stable, no alarm will be triggered. When a person moves within the detection area, there will be a change in the infrared thermal radiation energy. The infrared sensor converts the change in infrared thermal radiation energy between the moving human and the background objects into corresponding electrical signals, which are then processed appropriately and sent to the alarm controller to issue an alarm signal. 

The detection wavelength range of the infrared sensor is 8 to 14 μm. Since the infrared radiation wavelength of the human body falls within this detection wavelength range, it can effectively detect moving human bodies. The optical system in front of the infrared sensor can reflect or transmit the infrared radiation energy from multiple directions through a reflector or a special lens, and then converge all of it onto the infrared sensor. This not only improves the thermoelectric conversion efficiency of the infrared sensor but also extends the detection distance and expands the surveillance field of view.