PIR Sensor | Principle | 2 Important Applications



What are PIR sensors?

PIR sensor or Passive Infrared Sensor is an electronic instrument that detects the IR radiations emitted from objects present in its field of view. The term “passive” is added as a prefix because these types of sensors do not radiate energy in any form. These instruments are used for detecting the movement in objects, animals, and humans by using their infrared radiations. The emitted black body radiation (from a person, animal, or object) belonging in the mid-infrared wavelength range is sensed and compared with the temperature of the background objects by the detector.

An infrared wave is an electromagnetic radiation having a frequency between 300 GHz and 400 THz [wavelengths ranging from 10-3 – 0.7 x 10-6m]. IR radiations are not visible to human eyes. Infrared waves find their application in heating food and television remotes, fiber optic cables, thermal imaging cameras, medical applications, motion detectors, etc. PIR sensors are mainly used for detecting and analyzing the motion of things or people.

PIR sensor
Motion detector mounted on a wall. Image source: CHGMotion detector, marked as public domain, more details on Wikimedia Commons

What is the operating principle of PIR sensors?

Every warm object including the human body radiates heat in the form of infrared radiation. IR radiations are not visible to human eyes. These radiations can only be detected by specially designed electronic devices. Passive Infrared sensors detect and then convert the incident light energy(in the form of infrared waves) into an electrical (electrons) signal. IR sensors are severely affected by water vapor because water absorbs the maximum portion of the radiation received. Due to this, infrared detectors are at times unable to provide accurate results when it is used in an outdoor environment. The amount of radiation varies depending upon the temperature and surface characteristics of the given objects.

Typically, these detectors are capable of sensing one radiation band in the 4.4-micrometer range and the other two bands in the range above and below the 4.4-micrometer spectrum. The distinguished detection of the two ranges allows the sensor to differentiate between the actual flames and the non-flame radiations that might have influenced the results. These sensors can provide more accurate flame detection results after ignoring the effects of the background radiation.

A demonstration of the sensitivity of a PIR sensor with respect to direction. Image source: AndreasCTMotion Detector with Beam PatternCC BY-SA 4.0

Lenses used in PIR sensors

The most popular PIR detector models use Fresnel lenses or mirror segments, that have an effective range of around 10 meters or 30 feet, and a field of view less than 180 degrees. These lenses are used for focusing the infrared radiations that are detected by the device.  Some advanced models have a much wider field of view of about 360 degrees. These types of lenses are mounted on the ceiling. Passive infrared sensors are capable of detecting variations in infrared energy over 30 meters or 100 feet. Parabolic mirrors are used in place of Fresnel lenses for effectively focusing the radiation energy.

What is the application of the PIR sensor in thermal imaging?

Thermal imaging infrared sensors are specifically designed to detect infrared radiations and form images. The images produced by thermal imaging depends on the amount of heat radiated by the object and the temperature difference between objects present in the foreground and background. Thermal imaging is best suited for image formation in extremely dark conditions. Apart from glasses, thermal imaging camera sensors are also available for capturing images in the dark. These instruments serve a wide range of applications in various different fields. The process of analyzing thermal imaging data is known as thermography.

What is the application of the PIR sensor in security systems?

PIR sensors are commonly used in various security systems. The PIR system is responsible for controlling a small relay. This relay connects the security circuit with a couple of electrical contacts attached to a detection input zone of the burglar alarm control panel.  In case there is an absence of motion detection or variation, the relay contact is closed or there is a ‘normally closed’ (NC) relay. In case there is a presence of motion detection or variation, the relay opens the circuit and triggers the alarm sending a signal.

Passive sensors detect the radiation reflections from the surroundings. These sensing devices usually do not use a transmitter. These instruments analyze the radiation variations from the surrounding objects and then direct an electric pulse or signal to the embedded microcomputer. This microcomputer then interacts with the mechanical part of the sensor based on the signal received. Sensor manufacturers often advise a calculated placement of the passive infrared detector-based alarm for reducing the chances of false alarm situations. It is advisable to not mount a sensor facing a window as it may create an interference issue.

PIR security switch. Image source: Z22Light switch with passive infrared sensorCC BY-SA 4.0

What are the materials used in PIR sensors?

IR sensors can be made up of different materials based on the requirements of the form of information it has to provide. Some commonly used materials in IR detectors are:

  • Mercury cadmium telluride (Known as MCT, HgCdTe)
  • Lead(II) sulfide (PbS)
  • Indium antimonide (InSb)
  • Indium gallium arsenide
  • Indium arsenide
  • Lithium tantalate (LiTaO3)
  • Lead selenide
  • Quantum Well Infrared Photodetector (QWIP)
  • Triglycine sulfate (TGS)
  • Platinum Silicite (PtSi)

Motion sensing technologies have been developed highly over the years. Usually, a certain combination of two or more motion-detecting technologies is utilized for obtaining the best possible results. The combination of Passive infrared technologies and computer vision software is recently gaining popularity in the field of motion-sensing. Such technologies seldom produce errors in results and also rechecks the motion that is recorded by the computer vision camera. These devices serve a wide range of applications in various fields.

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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