What Is a Passive Infrared Sensor?

A passive infrared sensor (PIR) is a type of optical device that detects light wavelengths beyond human vision. Where human beings can generally see light from 400 to 700 nanometer wavelengths, infrared wavelengths span across a very broad spectrum from about 740 nanometers to 300,000 nanometers or more at 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 range of remote sensing residential and commercial applications because the sensors are inexpensive, long-lasting, 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 technology is its portability. PIR sensors are often used because they are an electronic device that requires no continuous electrical power source to operate. They act as a sort of dormant switch that is gauged to ambient infrared light within their line of sight. When a new heat source crosses their path, such as that of a human being or animal, they respond as a switch, close a circuit, and turn on an alarm, a faucet, or other device.

Since they are capable of detecting such a wide range of the infrared spectrum, special filters or light-focusing fresnel lenses can be placed atop the sensor's optical detector to customize it as well. They can, therefore, be optimized to ignore objects like small animals or pets, and respond to larger objects like people. They also often incorporate tiny parabolic mirrors to extend their sensing range to a broader horizontal area for such uses as automatic light switches when someone steps into a darkened room. The fresnel lens incorporated into the passive infrared sensor is a curved plastic sheet of hundreds of minute focusing prisms, which can take weak light signals received from multiple angles and channel them all to one focal point on the sensor's detector for maximum reception.

The core component of every PIR is a pyroelectric unit. This is essentially a little metal mounting case for a crystal that responds to temperature changes by producing an electrical current. The sensor is calibrated to ignore the background radiation from infrared light in its environment, and to produce a signal only when that radiation level significantly increases.