Arduino HC-SR501 PIR Motion Sensor Module

How does a PIR sensor work?

All objects, including the human body, at temperatures above absolute zero (0 Kelvin / -273.15 °C) emit heat energy in the form of infrared radiation. The hotter an object is, the more radiation it emits. This radiation is not visible to the human eye because it is emitted at infrared wavelengths. The PIR sensor is specifically designed to detect such levels of infrared radiation.

The Pyroelectric Sensor

A pyroelectric sensor consists of a window with two rectangular slots and is made of a material (typically coated silicon) that allows infrared radiation to pass through. Behind the window, there are two separate infrared sensor electrodes, one responsible for producing the positive output and the other for producing the negative output.

The two electrodes are wired such that they cancel each other out. This is because we are looking for changes in IR levels and not ambient IR levels. That’s why when one half sees more or less IR radiation than the other, we get the output.