PIR Motion Sensor
A passive infrared sensor (PIR Motion Sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are most often used in PIR-based motion detectors. PIR Motion Sensors are commonly used in security alarms and automatic lighting applications.
PIR Motion Sensors detect general movement, but do not give information on who or what moved. For that purpose, an imaging IR sensor is required.
PIR Motion Sensors are commonly called simply “PIR”, or sometimes “PID”, for “passive infrared detector”. The term passive refers to the fact that PIR devices do not radiate energy for detection purposes. They work entirely by detecting infrared radiation (radiant heat) emitted by or reflected from objects.
All objects with a temperature above absolute zero emit heat energy in the form of radiation. Usually this radiation isn’t visible to the human eye because it radiates at infrared wavelengths, but it can be detected by electronic devices designed for such a purpose.
A PIR Motion Sensor can detect changes in the amount of infrared radiation impinging upon it, which varies depending on the temperature and surface characteristics of the objects in front of the sensor. When an object, such as a person, passes in front of the background, such as a wall, the temperature at that point in the sensor’s field of view will rise from room temperature to body temperature, and then back again.
The PIR Motion Sensor converts the resulting change in the incoming infrared radiation into a change in the output voltage, and this triggers the detection. 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.
PIR Motion Sensors come in many configurations for a wide variety of applications. 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.
Models with wider fields of view, including 360°, are available, typically designed to mount on a ceiling. Some larger PIRs are made with single segment mirrors and can sense changes in infrared energy over 30 meters (100 feet) from the PIR.
There are also PIRs designed with reversible orientation mirrors which allow either broad coverage (110° wide) or very narrow “curtain” coverage, or with individually selectable segments to “shape” the coverage.
Pairs of sensor elements may be wired as opposite inputs to a differential amplifier. 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. (Continuous high energy exposure may still be able to saturate the sensor materials and render the sensor unable to register further information.)
At the same time, this differential arrangement minimizes common-mode interference, allowing the device to resist triggering due to nearby electric fields. However, a differential pair of sensors cannot measure temperature in this configuration, and therefore is only useful for motion detection.
When a PIR Motion Sensor is configured in a differential mode, it specifically becomes applicable as a motion detector device. In this mode when a movement is detected within the “line of sight” of the sensor, a pair of complementary pulses are processed at the output pin of the sensor.
In order to implement this output signal for a practical triggering of a load such as a relay or a data logger, or an alarm, the differential signal is rectified using a bridge rectifier and fed to a transistorized relay driver circuit.
The contacts of this relay close and open in response to the signals from the PIR, activating the attached load across its contacts, acknowledging the detection of a person within the predetermined restricted area.
The PIR Motion Sensor is typically mounted on a printed circuit board containing the necessary electronics required to interpret the signals from the sensor itself. The complete assembly is usually contained within a housing, mounted in a location where the sensor can cover the area to be monitored.
The housing will usually have a plastic “window” through which the infrared energy can enter. Despite often being only translucent to visible light, infrared energy is able to reach the sensor through the window because the plastic used is transparent to infrared radiation.
The plastic window reduces the chance of foreign objects (dust, insects, etc.) from obscuring the sensor’s field of view, damaging the mechanism, and/or causing false alarms. 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. It may also serve as a focusing mechanism.
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