IR (infrared) sensors are devices whose principle of operation is based on tracking the level of IR radiation in the field of view of a sensor (usually pyroelectric). The signal at the sensor output monotonously depends on the level of IR radiation averaged over the sensor's field of view. When a person (or other massive object with a temperature higher than the background temperature) appears, the voltage rises at the output of the pyroelectric sensor. In order to determine whether an object is moving, the sensor uses an optical system - a Fresnel lens. Sometimes, instead of a Fresnel lens, a system of concave segment mirrors is used. Segments of the optical system (lenses or mirrors) focus IR radiation on a pyroelectric element, which emits an electric pulse. As the source of infrared radiation moves, it is captured and focused by different segments of the optical system, which forms several successive pulses. Depending on the sensor sensitivity setting, 2 or 3 pulses must be sent to the sensor pyroelement to output the final signal.