This chip takes the output of the sensor and does some minor processing on it to emit a digital output pulse from the analog sensor. It seems that most small hobbyist sensors use the BISS0001 ("Micro Power PIR Motion Detector IC"), undoubtedly a very inexpensive chip. Along with the pyroelectic sensor is a bunch of supporting circuitry, resistors and capacitors. If one half sees more or less IR radiation than the other, the output will swing high or low. The two halves are wired up so that they cancel each other out. The reason for that is that we are looking to detect motion (change) not average IR levels. The sensor in a motion detector is actually split in two halves. Everything emits some low level radiation, and the hotter something is, the more radiation is emitted. PIRs are basically made of a pyroelectric sensor (which you can see above as the round metal can with a rectangular crystal in the center), which can detect levels of infrared radiation. They are often referred to as PIR, "Passive Infrared", "Pyroelectric", or "IR motion" sensors. For that reason they are commonly found in appliances and gadgets used in homes or businesses. They are small, inexpensive, low-power, easy to use and don't wear out. Pyroelectric ("Passive") InfraRed sensors: '''What is a PIR sensor?''' PIR sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range.
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