Operating Principles
All objects above absolute zero emit heat energy in the form of optical radiation (light). Usually this light is invisible to the human eye because body temperature radiates at infrared wavelengths, but it can be detected by electronic devices designed for such a purpose.
The term passive in this instance refers to the fact that PIR devices do not generate or radiate any energy for detection purposes. They work entirely by detecting the energy given off by other objects.
Read more about this topic: Passive Infrared Sensor
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