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Passive infrared (PIR) sensors detect infrared wavelengths beyond human vision and are used in security systems, motion-activated controls, and other technologies. They are portable, durable, and reliable, and do not require a continuous power source. Filters and lenses can be added to customize their detection range and ignore certain objects. The main component is a pyroelectric unit that produces an electric current in response to temperature changes.
A passive infrared (PIR) sensor is a type of optical device that detects wavelengths of light beyond human vision. Where humans can generally see light from 400 to 700 nanometers in wavelength, infrared wavelengths span a very broad spectrum from approximately 740 nanometers to 300,000 nanometers or more in the far infrared range. Most passive infrared sensor units, especially those built into security systems, have optical filters to limit their reception to between 8,000 and 14,000 nanometers, which is the range at which human bodies glow in the infrared. PIR sensor technology has been employed in a wide variety of residential and commercial remote sensing applications because the sensors are inexpensive, durable, and very reliable.
A primary feature of the passive infrared sensor that makes it popular in security systems, motion-activated controls such as on public faucets in restrooms, and other interactive technologies is its portability. PIR sensors are often used because they are an electronic device that does not require a continuous electrical power source to operate. They act as a sort of sleeper switch that is measured to ambient infrared light within their line of sight. When a new heat source crosses their path, such as that of a human or animal, they respond as a switch, closing a circuit and turning on an alarm, faucet or other device.
Because they are able to detect such a wide range of the infrared spectrum, special filters or Fresnel lenses for focusing light can be placed over the sensor’s optical detector to customize it. They can, therefore, be optimized to ignore objects such as small animals or pets and respond to larger objects such as people. They also often incorporate tiny parabolic mirrors to extend their detection range to a larger horizontal area for uses such as automatic light switches when someone enters a dark room. The Fresnel lens embedded in the passive infrared sensor is a curved plastic sheet of hundreds of minute focusing prisms, which can take faint light signals received from multiple angles and funnel them all into a focal point on the sensor’s detector for maximum reception.
The main component of any PIR is a pyroelectric unit. It is essentially a small metal mounting case for a crystal that responds to changes in temperature by producing an electric current. The sensor is calibrated to ignore background radiation of infrared light in its environment and to produce a signal only when that radiation level increases significantly.
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