Passive infrared sensor

Designrelay controls a set of electrical contacts that are
Infrared radiation enters through the front of theusually connected to the detection input of a burglar
sensor, known as the sensor face. At the core of aalarm control panel. If the amount of infrared energy
PIR sensor is a solid state sensor or set of sensors,focused on the pyroelectric sensor changes within a
made from an approximately 1/4 inch square ofconfigured time period, the device will switch the
natural or artificial pyroelectric materials, usually in thestate of the alarm relay. The alarm relay is typically a
form of a thin film, out of gallium nitride (GaN),"normally closed (NC)" relay, also known as a "Form
caesium nitrate (CsNO3), polyvinyl fluorides,B" relay.
derivatives of phenylpyrazine, and cobaltA person entering a monitored area is detected
phthalocyanine. (See pyroelectric crystals.) Lithiumwhen the infrared energy emitted from the intruder's
tantalate (LiTaO3) is a crystal exhibiting bothbody is focused by a Fresnel lens or a mirror
piezoelectric and pyroelectric properties.segment and overlaps a section on the chip that had
The sensor is often manufactured as part of anpreviously been looking at some much cooler part of
integrated circuit and may consist of one (1), two (2)the protected area. That portion of the chip is now
or four (4) 'pixels' of equal areas of the pyroelectricmuch warmer than when the intruder wasn't there.
material. Pairs of the sensor pixels may be wired asAs the intruder moves, so does the hot spot on the
opposite inputs to a differential amplifier. In such asurface of the chip. This moving hot spot causes the
configuration, the PIR measurements cancel eachelectronics connected to the chip to de-energize the
other so that the average temperature of the fieldrelay, operating its contacts, thereby activating the
of view is removed from the electrical signal; andetection input on the alarm control panel.
increase of IR energy across the entire sensor isConversely, if an intruder were to try to defeat a
self-cancelling and will not trigger the device. ThisPID, perhaps by holding some sort of thermal shield
allows the device to resist false indications of changebetween himself and the PID, a corresponding 'cold'
in the event of being exposed to flashes of light orspot moving across the face of the chip will also
field-wide illumination. (Continuous bright light could stillcause the relay to de-energize unless the thermal
saturate the sensor materials and render the sensorshield has the same temperature as the objects
unable to register further information.) At the samebehind it.
time, this differential arrangement minimizesManufacturers recommend careful placement of their
common-mode interference, allowing the device toproducts to prevent false (non-intruder caused)
resist triggering due to nearby electric fields.alarms. They suggest mounting the PIDs in such a
However, a differential pair of sensors cannotway that the PID cannot 'see' out of a window.
measure temperature in that configuration andAlthough the wavelength of infrared radiation to
therefore this configuration is specialized for motionwhich the chips are sensitive does not penetrate
detectors, see below.glass very well, a strong infrared source such as
PIR-based motion detectorfrom a vehicle headlight or sunlight reflecting from a
Cylindrical facet lens in front of PIR sensor. Eachvehicle window can overload the chip with enough
facet (rectangle) is a Fresnel lensinfrared energy to fool the electronics and cause a
In a PIR-based motion detector (usually called a PID,false alarm. A person moving on the other side of
for Passive Infrared Detector), the PIR sensor isthe glass however would not be 'seen' by the PID.
typically mounted on a printed circuit board containingThey also recommended that the PID not be placed
the necessary electronics required to interpret thein such a position that an HVAC vent would blow hot
signals from the pyroelectric sensor chip. Theor cold air onto the surface of the plastic which
complete assembly is contained within a housingcovers the housing's window. Although air has very
mounted in a location where the sensor can view thelow emissivity (emits very small amounts of infrared
area to be monitored. Infrared energy is able toenergy), the air blowing on the plastic window cover
reach the pyroelectric sensor through the windowcould change the plastic's temperature enough to,
because the plastic used is transparent to infraredonce again, fool the electronics.
radiation (but only translucent to visible light). ThisPIDs come in many configurations for a wide variety
plastic sheet also prevents the intrusion of dust andof applications. The most common, used in home
or insects from obscuring the sensor's field of view,security systems, have numerous Fresnel lenses or
and in the case of insects, from generating falsemirror segments and an effective range of about
alarms.thirty feet. Some larger PIDs are made with single
A few mechanisms have been used to focus thesegment mirrors and can sense changes in infrared
distant infrared energy onto the sensor surface. Theenergy over one hundred feet away from the PID.
window may have multiple Fresnel lenses molded intoThere are also PIDs designed with reversible
it.orientation mirrors which allow either broad coverage
Multi-Fresnel lens type of PID(110 wide) or very narrow 'curtain' coverage.
Typical residential and/or commercial PID withPIDs can have more than one internal sensing
multi-Fresnel lens cover.element so that, with the appropriate electronics and
PID front cover only with point light source behind toFresnel lens, it can detect direction. Left to right, right
show individual lenses.to left, up or down and provide an appropriate
PID with front cover removed showing location ofoutput signal.
pyroelectric sensor (green arrow).PIR-based remote thermometer
Alternatively, some PIDs are manufactured withDesigns have been implemented in which a PIR circuit
internal plastic, segmented parabolic mirrors to focusmeasures the temperature of a remote object. In
the infrared energy. Where mirrors are used, thesuch a circuit, a non-differential PIR output is used.
plastic window cover has no Fresnel lenses moldedThe output signal is evaluated according to a
into it. This filtering window may be used to limit thecalibration for the IR spectrum of a specific type of
wavelengths to 8-14 micrometers which is closest tomatter to be observed. By this means, relatively
the infrared radiation emitted by humans (9.4accurate and precise temperature measurements
micrometers being the strongest).may be obtained remotely. Without calibration to the
Segmented mirror type of PIDtype of material being observed, a PIR thermometer
Typical residential and/or commercial PID using andevice is able to measure changes in IR emission
internal segmented mirror for focusing.which correspond directly to temperature changes,
Cover removed. Segmented mirror at bottom withbut the actual temperature values cannot be
PC (printed circuit) board above it.calculated.
Printed circuit board removed to show segmentedSee also
mirror.List of sensors
Segmented parabolic mirror removed from housing.Infrared point sensor
Rear of circuit board which faces mirror when inHeat detector
place. Pyroelectric sensor indicated by green arrow.Notes
The PID can be thought of as a kind of infrared^ "PIR Motion Sensor" page of
camera that remembers the amount of infrared^ C. F. Tsai and M. S. Young (December 2003).
energy focused on its surface. Once power is applied"Pyroelectric infrared sensor-based thermometer for
to the PID, the electronics in the PID shortly settlemonitoring indoor objects". Review of Scientific
into a quiescent state and energize a small relay. ThisInstruments 74 (12): 52675273. doi:10.1063/1.1626005.