Passive infrared sensor
A passive infrared sensor (PIR 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 sensors are commonly used in security alarms and automatic lighting applications.
PIR sensors detect general movement, but do not give information on who or what moved. For that purpose, an imaging IR sensor is required.
PIR 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
Operating principles
All objects with a temperature above absolute zero emit heat energy in the form of electromagnetic 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.
PIR-based motion detector
A PIR-based
Operation
A PIR 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.
PIRs 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.
Differential detection
Pairs of sensor elements may be wired as opposite inputs to a
Practical Implementation
When a PIR 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[4] 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.
Product design
The PIR 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
Focusing
Different mechanisms can be used to focus the distant infrared energy onto the sensor surface.
Lenses
The plastic window covering may have multiple facets molded into it, to focus the infrared energy onto the sensor. Each individual facet is a Fresnel lens.
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PIR motion detector housing with cylindrical faceted window. The animation highlights individual facets, each of which is a Fresnel lens, focusing light on the sensor element underneath.
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PIR front cover only (electronics removed), with point light source behind, to show individual lenses.
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PIR with front cover removed, showing location of pyroelectric sensor (green arrow).
Mirrors
Some PIRs are manufactured with internal, segmented
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Typical residential/commercial PID using an internal segmented mirror for focusing.
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Cover removed. Segmented mirror at bottom with PC (printed circuit) board above it.
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Printed circuit board removed to show segmented mirror.
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Segmented parabolic mirror removed from housing.
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Rear of circuit board which faces mirror when in place. Pyroelectric sensor indicated by green arrow.
Beam pattern
As a result of the focussing, the detector view is actually a beam pattern. Under certain angles (zones), the PIR sensor receives almost no radiation energy and under other angles the PIR receives concentrated amounts of infrared energy. This separation helps the motion detector to discriminate between field-wide illumination and moving objects.
When a person walks from one angle (beam) to another, the detector will only intermittently see the moving person. This results in a rapidly changing sensor signal which is used by the electronics to trigger an alarm or to turn on lighting. A slowly changing signal will be ignored by the electronics.
The number, shape, distribution and sensitivity of these zones are determined by the lens and/or mirror. Manufacturers do their best to create the optimal sensitivity beam pattern for each application.
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Motion detector with superimposed beam pattern. The length of the beams is a measure of the detectors sensitivity in that direction.
Automatic lighting applications
When used as part of a lighting system, the electronics in the PIR typically control an integral relay capable of switching mains voltage. This means the PIR can be set up to turn on lights that are connected to the PIR when movement is detected. This is most commonly used in outdoor scenarios either to deter criminals (security lighting) or for practical uses like the front door light turning on so you can find your keys in the dark.
Additional uses can be in public toilets, walk-in pantries, hallways or anywhere that automatic control of lights is useful. This can provide energy savings as the lights are only turned on when they are needed and there is no reliance on users remembering to turn the lights off when they leave the area.
Security applications
When used as part of a security system, the electronics in the PIR typically control a small
Placement
Manufacturers recommend careful placement of their products to prevent false alarms (i.e., any detection not caused by an intruder).
They suggest mounting the PIRs in such a way that the PIR cannot "see" out of a window. Although the wavelength of infrared radiation to which the chips are sensitive does not penetrate glass very well, a strong infrared source (such as from a vehicle headlight or sunlight) can overload the sensor and cause a false alarm. A person moving on the other side of the glass would not be "seen" by the PID. That may be good for a window facing a public sidewalk, or bad for a window in an interior partition.
It is also recommended that the PIR not be placed in such a position that an
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.
Since PIR sensors have ranges of up to 10 meters (30 feet), a single detector placed near the entrance is typically all that is necessary for rooms with only a single entrance. PIR-based security systems are also viable in outdoor security and motion-sensitive lighting; one advantage is their low power draw, which allows them to be solar-powered.[5]
PIR remote-based thermometer
Designs have been implemented in which a PIR circuit measures the temperature of a remote object.[6] In such a circuit, a non-differential PIR output is used. The output signal is evaluated according to a calibration for the IR spectrum of a specific type of matter to be observed. By this means, relatively accurate and precise temperature measurements may be obtained remotely. Without calibration to the type of material being observed, a PIR thermometer device is able to measure changes in IR emission which correspond directly to temperature changes, but the actual temperature values cannot be calculated.
See also
- Heat detector
- Infrared point sensor
- Infrared sensor
- List of sensors
Notes
- ^ Product Specification for PR150-1L/PR180-1L. Leviton. Retrieved 6 September 2014.[permanent dead link]
- ^ "How Infrared motion detector components work". Non commercial research page. Glolab Corporation.
- ^ "PIR sensor technology". ecosirius.com. Retrieved 1 February 2014.
- ^ "PIR Sensor Output Pulse Generation".
- ^ D., Hallee. "Passive Infrared Sensors: A Brief Overview". InHomeSafetyGuide.org. In Home Safety Guide. Retrieved 6 May 2016.
- .
External links
- How Infrared motion detector components work
- Design advice and assembly instructions from a motion detector kit
- U.S. patent 3,703,718, Infrared Intrusion Detector System, issued Nov 21 1972 to Herbert L. Berman, contains a very clear explanation