Typical residential/commercial PIR-based motion detector (PID).
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 active 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 infrared radiation (radiant heat) emitted by or reflected from objects.
Operating principles
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.
Construction
Infrared radiation enters through the front of the sensor, known as the 'sensor face'. At the core of a PIR sensor is a solid state sensor or set of sensors, made from pyroelectric materials—materials which generate energy when exposed to heat. Typically, the sensors are approximately 1/4 inch square (40 mm2), and take the form of a thin film. Materials commonly used in PIR sensors include gallium nitride (GaN), caesium nitrate (CsNO3), polyvinyl fluorides, derivatives of phenylpyridine, and cobalt phthalocyanine. The sensor is often manufactured as part of an integrated circuit.
PIR-based motion detector
A PIR motion detector used to control an outdoor, automatic light.
An indoor light switch equipped with PIR-based occupancy sensor
A PIR-based motion detector is used to sense movement of people, animals, or other objects. They are commonly used in burglar alarms and automatically-activated lighting systems.
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.
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 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.
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 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.
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
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.
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PIR motion sensor design
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; see below.
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.
- Multi-Fresnel lens type of PIR
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.
PIR front cover only (electronics removed), with point light source behind, to show individual lenses.
PIR with front cover removed, showing location of pyroelectric sensor (green arrow).
Mirrors
Some PIRs are manufactured with internal, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover generally has no Fresnel lenses molded into it.
- Segmented mirror type of PIR
Typical residential/commercial PID using an internal segmented mirror for focusing.
Cover removed. Segmented mirror at bottom with PC (printed circuit) board above it.
Printed circuit board removed to show segmented mirror.
Segmented parabolic mirror removed from housing.
Rear of circuit board which faces mirror when in place. Pyroelectric sensor indicated by green arrow.
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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.
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 relay. This relay completes the circuit across a pair of electrical contacts connected to a detection input zone of the burglar alarm control panel.
The system is usually designed such that if no motion is being
detected, the relay contact is closed—a 'normally closed' (NC) relay. If
motion is detected, the relay will open the circuit, triggering the
alarm; or, if a wire is disconnected, the alarm will also operate.
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 HVAC vent would blow hot or cold air onto the surface of the plastic which covers the housing's window. Although air has very low emissivity
(emits very small amounts of infrared energy), the air blowing on the
plastic window cover could change the plastic's temperature enough to
trigger a false alarm.
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.
PIR remote-based thermometer
Designs have been implemented in which a PIR circuit measures the temperature of a remote object.
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.
