How does the PIR sensor work

　　A PIR sensor is an electronic sensor that measures infrared (IR) light radiated from an object in its field of view. They are most commonly used in PIR-based motion detectors. All bodies with temperatures above absolute zero give off heat energy in the form of radiation. Normally this radiation is invisible to the human eye because it radiates at infrared wavelengths, but can be detected by electronic equipment designed for this purpose.

　　TPA81 is a thermopile array that can detect infrared in the range of 2um-22um. This is the wavelength of radiant heat. Pyro-electric sensors are commonly used in burglar alarms and turn on external lights to detect infrared in the same band. These thermoelectric sensors can only detect changes in heat -- so they're motion detectors. Although useful in robotics, their application is limited by their inability to detect and measure the temperature of a static heat source. Another type of sensor is the thermopile array. These are used for non-contact infrared thermometers. They have a very wide detection Angle or field of view (FOV) of about 100° and require masking or lenses or often both to obtain a more useful FOV of about 12°. Some have built-in lenses. More recently, sensors with thermonile arrays, built-in electronics and silicon lenses have been marketed. This is the type used in TPA81. It has eight rows of thermopile. TPA81 can measure the temperature of eight adjacent points simultaneously. The TPA81 can also control servos to translate modules and build thermal images. TPA81 can detect candle flames up to 2 meters (6 ft) away from ambient light!

　　PIR sensors are similar to cameras that can only see warm objects. So TPA81 is a thermal imager with a resolution of 8 pixels. Of course, the human body always heats up. Now, if we control the PIR sensor through the controller and define some gestures for the PIR sensor, we can make a simple gesture detector.

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