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.
The PIR sensor is designed such that the light enters through the front of the sensor, which is known as the sensor face. This sensor face usually is made up of smaller sensor components which basically consist of pyroelectric materials in the form of thin films. These thin films may be made of Caesium nitrate or cobalt phthalocyanine and since these materials are pyroelectric they convert the light energy falling on them into small electrical signals. The IR sensor is contained within a fully sealed metal box so as to improve immunity towards noise, humidity, temperature and other effects. The window is the face, which is always made of IR-transmissive substances and is typically covered with silicon which is not only cheap but keeps the sensing element protected. At the rear of the window the balanced sensors are situated.
The device usually has 2 sensors connected to a differential amplifier so that useless signals are cancelled out and only valid inputs are detected. Though the range of PIR sensors is typically up to few 10s of feet but with some modifications, larger detectors using mirrors can sense changes in infrared signals up to a hundred feet away.
These sensors are often used to make other devices as well like PIR based remote thermometers; here a non differential PIR output is used. As the temperature of the body increases the radiation frequency changes and the sensor is able to detect that and a simple circuit attached to the output if properly calibrated can easily read the body’s temperature. In modern robotics such sensors are often installed in autonomous bots for motion sensing and path detection like in the case of line followers.