Passive Infrared (PIR) sensors are electronic devices that detect infrared radiation emitted by living organisms and other sources of heat. They are widely used in security systems, motion detection systems, and automatic lighting systems. The principle behind PIR sensors is based on the fact that all objects with a temperature above absolute zero emit infrared radiation.
When an object is in the field of view of a PIR sensor, it absorbs some of the infrared radiation emitted by the sensor, causing a change in the temperature of the object. The sensor detects this change in temperature and generates an electrical signal, which is used to trigger an alarm or activate a light.
The PIR sensor consists of a detector, which is typically made of a pyroelectric material, and a lens that focuses the infrared radiation onto the detector. The pyroelectric material has the property of generating a voltage when its temperature changes. When the temperature of the material changes due to the absorption of infrared radiation, the voltage across the material changes, which is detected by the circuitry of the sensor.
The lens of the PIR sensor is designed to focus the infrared radiation from the field of view onto the detector. The lens is made of a material that is transparent to infrared radiation and has a special shape that allows the sensor to detect motion in a particular direction. The shape of the lens also determines the range of the sensor, which is typically up to a few meters.
The sensor is designed to be sensitive to changes in the infrared radiation emitted by living organisms, such as humans and animals, but not to changes in the background temperature, such as the temperature of the air or walls. This is achieved by using two or more detectors that are connected in a differential configuration. One detector is exposed to the field of view, while the other detectors are shielded from the radiation. The difference in the signal from the exposed detector and the shielded detectors is used to cancel out the background temperature and detect only the changes due to the presence of living organisms.
The output of the PIR sensor is typically a digital signal that is sent to a microcontroller or a processing unit that can interpret the signal and take appropriate actions, such as triggering an alarm or activating a light. The microcontroller can also be programmed to ignore signals that are caused by the movement of non-living objects, such as curtains or plants.
In conclusion, PIR sensors work by detecting changes in the infrared radiation emitted by living organisms and other sources of heat. They are widely used in security systems, motion detection systems, and automatic lighting systems. The sensor consists of a pyroelectric detector and a lens that focuses the radiation onto the detector. The sensor is designed to be sensitive to changes in the radiation from living organisms, but not to changes in the background temperature. The output of the sensor is typically a digital signal that is sent to a microcontroller or a processing unit that can interpret the signal and take appropriate actions.