PIR(Passive InfraRed) sensor'
What is PIR sensor?'
Infrared radiation exists in the electromagnetic spectrum at a wavelength that is longer than visible light.
It cannot be seen but it can be detected. Objects that generate heat also generate infrared radiation and
those objects include animals and the human body whose radiation is strongest at a wavelength of 9.4um.
Infrared in this range will not pass through many types of material that pass visible light such as ordinary
window glass and plastic. However it will pass through, with some attenuation, material that is opaque to
visible light such as germanium and silicon. An unprocessed silicon wafer makes a good IR window in a
weatherproof enclosure for outdoor use. It also provides additional filtering for light in the visible range.
The PIR Sensor detects motion up to 20 feet away by using a Fresnel lens and infrared-sensitive element
to detect changing patterns of passive infrared emitted by objects in its vicinity.
How does PIR sensor work as a motion detecting sensor?'
An IR emitting body moving across the front of a sensor will expose first one, then both and then the
other sensor element.The output signal waveform shows that first a positive, then zero and then a
negative transition results.There are many different ways to create a motion sensor.
Motion is detected when an infrared emitting source with one temperature, such as a human body,
passes in front of a source with another temperature, such as a wall.
Infrared radiation enters through the front of the sensor, known as the sensor face. At the core
of a PIR is a solid state sensor or set of sensors, made from approximately 1/4 inches 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.
A Fresnel lens is a Plano Convex lens that has been collapsed on itself to form a flat lens that retains
its optical characteristics but is much thinner and therefore has less absorption loss.
A Fresnel lens is usually thin and flexible and is about 0.015 inch (0.38mm) thick with grooves molded
into one surface. The groove side of the lens usually faces the PIR sensor. A Fresnel lens both captures
more IR radiation and focuses it to a small point. This focal point moves across the sensor as the IR source
moves and exposes one element at a time. A Fresnel lens can extend detection range to about 100 feet.
Because the sensor is passive, it relies on an infrared signature (heat) from the object to be monitored
so only warm-bodied items (relative to the ambient temperature of the room) can be detected.
For example, people and animals can be detected but a ball would not -unless it was noticeably
warmer or colder than the ambient temperature.
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