![]() If there's a datasheet, you'll want to refer to it Nearly all PIRs will have slightly different specifications, although they all pretty much work the same. Experimentation is key! Some basic stats These stats are for the PIR sensor in the Adafruit shop which is very much like the Parallax one. Note that PIRs won't tell you how many people are around or how close they are to the sensor, the lens is often fixed to a certain sweep and distance (although it can be hacked somewhere) and they are also sometimes set off by house pets. They are low power and low cost, pretty rugged, have a wide lens range, and are easy to interface with. For many basic projects or products that need to detect when a person has left or entered the area, or has approached, PIR sensors are great. 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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