These devices are centred around two balanced strips of a pyroelectric material which are placed parallel to one another, creating a pyroelectric sensor. This detects thermal energy in the surrounding area. An infrared filter is also included to block out all other light wavelengths.
A Fresnel lens helps to focus the infrared energy on the sensor and increases the sensing range. This is achieved by the lens collecting light from multiple angles and focusing it into a single point.
If a change to the signal differential between the two pyroelectric elements is detected, the PIR sensor engages and sends an electronic signal to an embedded computer. Several things could happen in this instance depending on the setup, such as triggering an alarm or activating a security light.
Crucially, a PIR motion detector will not determine the specific location of an object. Instead, it merely tells you that an object has entered the detection area.