PIR (passive infrared) sensors utilise the detection of infrared that is radiated from all objects that emit heat. This type of emission is not visible to the human eye, but sensors that operate using infrared wavelengths can detect such activity. They are sometimes referred to as ‘motion-based detectors’, as they sense the presence of people, animals and objects through the movement of their infrared wavelengths.
They are referred to as “passive” due to the fact that no heat or energy is emitted by the sensor itself. It is also important to remember that PIR sensors detect the emission of infrared radiation, and not heat.