Infrared is the invisible radiation lies between the Visible and Microwave portions of the electromagnetic spectrum. The term Infra means below the ability to detect visually and the term Red means colour of the light with low energy level. The Near infrared is close to the red portion of visible light and the Far infrared is close to the microwave region. Far infrared is thermal and we feel it as heat by the temperature sensors in our skin. Short and Near infrared rays cannot be detected since these are invisible as well as cool. Active infrared rays are produced from semiconductor sources but natural objects like animals including human beings emit infrared rays from the body heat. Humans at normal body temperature emits Infrared rays at the wavelength of 10 microns.PIR Sensors are used in surveillance systems to detect moving persons in protected areas.
Infrared spectrum is divided into four divisions based on the wavelength.
- Near infrared ( NIR)- 0.7-1 micrometers
- Short wave infrared – 1-3 micrometers
- Mid wave infrared – 3-5 micrometers
- Long wave infrared – 8-14 micrometers
The common Silicon detectors are sensitive to about 1050 nm, while the Indium Gallium Arsenide(InGaAs) is sensitive between 950 nm to 2600 nm.
Thermoreception is the sense of temperature detection seen in animals. Many animals can sense variations in atmospheric temperature as well as temperature of the nearby objects or organisms. The most advanced form of thermoreceptors are seen in Pit Viper, the most poisonous snake. It has theromosensors called Loreal organs located in pits presents above the nose. When the Loreal organ gets infrared from a warm blooded organism, the heat causes formation of ion channels in the sensory membrane which triggers the nerves. Vampire Bats (Blood sucking) also have infrared sensors in the nose to detect warm blooded prey. Fire seeking beetles, some butterflies and beetles also have sensory structures to detect infrared rays.
Human beings cannot detect infrared rays but emits infrared from the body like all other warm blooded animals( animals in which body temperature remains constant).This is exploited in Motion detectors to detect the presence of human beings.PIR sensors( Passive Infrared sensor) are used to activate an alarm system using the Infrared from the human body. PIR sensor is an electronic device that measures the infrared in its field of view. Movement is detected by the sensor when an infrared source like human passes in front of the sensor with one temperature and comparing another temperature like that of the wall in which the sensor is mounted.
The name PIR (Passive Infrared) is given to the sensor because it receives the infrared rays passively and do not emit any infrared ray. The PIR sensor is a solid state device made of ¼ inch pyroelectric material as a thin film. The pyroelectric material may be Gallium nitride(GaN), Cesium nitrate(CsNO3), Cobalt phthalocyanine, Lithium tantalite(LiTaO3) etc. They show both piezoelectric and pyroelectric properties. The sensor is made as part of the integrated circuit having 1 – 4 pixels of pyroelectric material. The paired pixels are connected to the inputs of a differential amplifier. The differential amplifier cancel each other the PIR measurements and the average temperature in the field of view is removed from the electrical signal. So that the IR energy on the sensor will not trigger the alarm. This prevents false triggering when exposed to flashes of light. The differential amplifier also minimize the common mode interference from electric fields.
The PIR sensor is mounted on a PCB with other semiconductors. The complete assembly is mounted in a case with a Fresnel lens molded in front of it. Behind the lens, there is a small window through which infrared enters into the sensor. The window is covered with a transparent plastic which allows only IR rays to enter into the sensor and prevents visible light. The filtering window limits the IR rays to 8-14 micrometers similar to the IR rays from human body.
The circuit in the PIR sensor is connected to a relay and the sensor resets at power on and remains standby. If the amount of the IR in front of the sensor changes within a configured time, the circuit switches off the relay to trigger the alarm system.
When a person enters within the range of the Sensor, The Fresnel lens focus the IR energy into the sensor which had previously sensed the cooler area. This change in energy level makes the sensor warmer and makes a hot spot in it. When the person moves, the hot spot also moves which causes the electronic circuit to deactivate the relay. This activates the alarm system connected through the normally closed(NC) contacts of the relay. If the person tries to mask the sensor with some shield, the sensor develops a cold spot which also activate the alarm. Thus the PIR sensor is fail proof.
Fixing and focusing the PIR sensor is important to give maximum sensitivity and to prevent false alarms. Mounting the device on the wall at an angle of 45 degree and 2-3 meters height will give good results. The sensor may saturate if strong sunlight and head light from vehicles fall on the sensor. Such positions should be avoided.