Passive infrared sensor
The term passive refers to the fact that PIR devices do not radiate energy for detection purposes.
Objects of similar temperature but different surface characteristics may also have a different infrared emission pattern, and thus moving them with respect to the background may trigger the detector as well.
The most common models have numerous Fresnel lenses or mirror segments, an effective range of about 10 meters (30 feet), and a field of view less than 180°.
In such a configuration, the PIR measurements cancel each other so that the average temperature of the field of view is removed from the electrical signal; an increase of IR energy across the entire sensor is self-cancelling and will not trigger the device.
This allows the device to resist false indications of change in the event of being exposed to brief flashes of light or field-wide illumination.
At the same time, this differential arrangement minimizes common-mode interference, allowing the device to resist triggering due to nearby electric fields.
The complete assembly is usually contained within a housing, mounted in a location where the sensor can cover the area to be monitored.
The window may be used as a filter, to limit the wavelengths to 8-14 micrometres, which is closest to the infrared radiation emitted by humans.
This results in a rapidly changing sensor signal which is used by the electronics to trigger an alarm or to turn on lighting.
When used as part of a lighting system, the electronics in the PIR typically control an integral relay capable of switching mains voltage.
This relay completes the circuit across a pair of electrical contacts connected to a detection input zone of the burglar alarm control panel.
Manufacturers recommend careful placement of their products to prevent false alarms (i.e., any detection not caused by an intruder).
It is also recommended that the PIR not be placed in such a position that an HVAC vent would blow hot or cold air onto the surface of the plastic which covers the housing's window.
Sensors are also often designed to "ignore" domestic pets, such as dogs or cats, by setting a higher sensitivity threshold, or by ensuring that the floor of the room remains out of focus.
The output signal is evaluated according to a calibration for the IR spectrum of a specific type of matter to be observed.
