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PIR stands for the passive infrared sensor.
It is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view.
They are most often used in PIR-based motion detectors.
All objects with a temperature above absolute zero emit heat energy in the form of radiation.
Usually, this radiation isn’t visible to the human eye because it 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 energy for detection purposes.
They work entirely by detecting infrared radiation emitted by or reflected from objects.
They do not detect or measure “heat”.
For establishing wireless communication with a remote device, usually, we use radio waves, optical radiations, and sometimes acoustic waves.
Basically, these types of wireless communications change their frequencies.
All these communications have variable frequencies starting from bands like HF, LF, VHF, UHF bands, and so on.
Optical radiations use an infrared and visible section of the spectrum; acoustic waves use an ultrasonic portion of the frequency spectrum; and, micro and millimeter waves are referred to as radio waves.