Passive infrared sensor - Principle and construction


Passive infrared sensor / PIR sensor 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. PIR sensors are commonly used in security alarms and automatic lighting applications.PIR sensors detect general movement, but do not give information on who or what moved. For that purpose, an active IR sensor is required.PIR sensors are commonly called simply "PIR", or sometimes "PID", for "passive infrared detector". The term passive refers to the fact that PIR devices do not radiate energy for detection purposes. They work entirely by detecting infrared radiation (radiant heat) emitted by or reflected from objects.


The 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 PIR sensor consists of a pyroelectric sensor and a Fresnel lens. This curved lens concentrates infrared radiation toward a detector’s sensor. The sensor output is inverted by the transistor. Collector of the transistor is connected to the input pin forms the latch circuit which is set when PIR output goes high to indicate the presence of a warm body. Output of the latch pin operates the relay driving circuit formed by transistors arranged in emitter follower mode.


A PIR sensor can detect changes in the amount of infrared radiation impinging upon it, which varies depending on the temperature and surface characteristics of the objects in front of the sensor. When an object, such as a person, passes in front of the background, such as a wall, the temperature at that point in the sensor's field of view will rise from room temperature to body temperature, and then back again. The sensor converts the resulting change in the incoming infrared radiation into a change in the output voltage, and this triggers the detection. Objects of similar temperature but different surface characteristics may also have a different infrared emission pattern, and thus moving them with respect to the background may trigger the detector as well.