Passive infrared sensor

Design

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 an approximately 1/4 inch square of natural or artificial pyroelectric materials, usually in the form of a thin film, out of gallium nitride (GaN), caesium nitrate (CsNO3), polyvinyl fluorides, derivatives of phenylpyrazine, and cobalt phthalocyanine. (See pyroelectric crystals.) Lithium tantalate (LiTaO3) is a crystal exhibiting both piezoelectric and pyroelectric properties.

The sensor is often manufactured as part of an integrated circuit and may consist of one (1), two (2) or four (4) ‘pixels’ of equal areas of the pyroelectric material. Pairs of the sensor pixels 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 flashes of light or field-wide illumination. (Continuous bright light could still 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 that configuration and therefore this configuration is specialized for motion detectors, see below.

PIR-based motion detector

Cylindrical facet lens in front of PIR sensor. Each facet (rectangle) is a Fresnel lens

In a PIR-based motion detector (usually called a PID, for Passive Infrared Detector), the PIR sensor is typically mounted on a printed circuit board containing the necessary electronics required to interpret the signals from the pyroelectric sensor chip. The complete assembly is contained within a housing mounted in a location where the sensor can view the area to be monitored. Infrared energy is able to reach the pyroelectric sensor through the window because the plastic used is transparent to infrared radiation (but only translucent to visible light). This plastic sheet also prevents the intrusion of dust and/or insects from obscuring the sensor’s field of view, and in the case of insects, from generating false alarms.

A few mechanisms have been used to focus the distant infrared energy onto the sensor surface. The window may have multiple Fresnel lenses molded into it.

Multi-Fresnel lens type of PID

Typical residential and/or commercial PID with multi-Fresnel lens cover.

PID front cover only with point light source behind to show individual lenses.

PID with front cover removed showing location of pyroelectric sensor (green arrow).

Alternatively, some PIDs are manufactured with internal plastic, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover has no Fresnel lenses molded into it. This filtering window may be used to limit the wavelengths to 8-14 micrometers which is closest to the infrared radiation emitted by humans (9.4 micrometers being the strongest).

Segmented mirror type of PID

Typical residential and/or 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.

The PID can be thought of as a kind of infrared camera that remembers the amount of infrared energy focused on its surface. Once power is applied to the PID, the electronics in the PID shortly settle into a quiescent state and energize a small relay. This relay controls a set of electrical contacts that are usually connected to the detection input of a burglar alarm control panel. If the amount of infrared energy focused on the pyroelectric sensor changes within a configured time period, the device will switch the state of the alarm relay. The alarm relay is typically a “normally closed (NC)” relay, also known as a “Form B” relay.

A person entering a monitored area is detected when the infrared energy emitted from the intruder’s body is focused by a Fresnel lens or a mirror segment and overlaps a section on the chip that had previously been looking at some much cooler part of the protected area. That portion of the chip is now much warmer than when the intruder wasn’t there. As the intruder moves, so does the hot spot on the surface of the chip. This moving hot spot causes the electronics connected to the chip to de-energize the relay, operating its contacts, thereby activating the detection input on the alarm control panel. Conversely, if an intruder were to try to defeat a PID, perhaps by holding some sort of thermal shield between himself and the PID, a corresponding ‘cold’ spot moving across the face of the chip will also cause the relay to de-energize unless the thermal shield has the same temperature as the objects behind it.

Manufacturers recommend careful placement of their products to prevent false (non-intruder caused) alarms. They suggest mounting the PIDs in such a way that the PID 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 reflecting from a vehicle window can overload the chip with enough infrared energy to fool the electronics and cause a false alarm. A person moving on the other side of the glass however would not be ‘seen’ by the PID.

They also recommended that the PID 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, once again, fool the electronics.

PIDs come in many configurations for a wide variety of applications. The most common, used in home security systems, have numerous Fresnel lenses or mirror segments and an effective range of about thirty feet. Some larger PIDs are made with single segment mirrors and can sense changes in infrared energy over one hundred feet away from the PID. There are also PIDs designed with reversible orientation mirrors which allow either broad coverage (110 wide) or very narrow ‘curtain’ coverage.

PIDs can have more than one internal sensing element so that, with the appropriate electronics and Fresnel lens, it can detect direction. Left to right, right to left, up or down and provide an appropriate output signal.

PIR-based remote 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.

See also

List of sensors

Infrared point sensor

Heat detector

Notes

^ “PIR Motion Sensor” page of GadgetShack.com.

^ C. F. Tsai and M. S. Young (December 2003). “Pyroelectric infrared sensor-based thermometer for monitoring indoor objects”. Review of Scientific Instruments 74 (12): 52675273. doi:10.1063/1.1626005. http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=RSINAK000074000012005267000001&idtype=cvips&gifs=yes. 

External links

How Infrared motion detector components work

Design advice and assembly instructions from a motion detector kit

Categories: Sensors | Optical devices | Security technology
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