PIR or the Passive Infrared sensor is an electronic device which is used to measure infrared light radiating from a body in its field of view. The sensors are known as Passive since they do not have an in built IR source like active sensors and detect the natural radiation of the body.
All objects have a certain body temperature and at any specific temperature there is a certain radiation that the body emits, this is called the Black Body Radiation phenomenon. The infrared radiation is undetectable to the human eye but can be detected by a PIR sensor. These sensors are commonly used as motion detector that is whenever a different body of different temperature passes from ahead of it, there is an apparent change in the infrared frequency detected due to different temperature of each body. This application is widely used in security equipments and other photo sensing equipment. The PIR sensors are very often paired with other devices to increase their range to encompass even human body sensors.