What is a passive infrared sensor?
A passive infrared sensor (PIR) is an electronic sensor that uses infrared radiation to detect variations in its field of view. They are most commonly used as motion sensors; for example, they are used to minimize power consumption by switching off lights and utilities if nobody is at home. They are also used in state-of-the-art burglar alarm systems to trigger a switch when motion is detected.
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Note
If you would like to learn more about how they work, you should refer to this page (https://learn.adafruit.com/pir-passive-infrared-proximity-motion-sensor/) at Adafruit. Adafruit and Ladyada are really good resources for building Arduino projects.