![]() 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. These change pulses are what is detected.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. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor, which causes a positive differential change between the two halves. When the sensor is idle, both slots detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. The lens used here is not really doing much and so we see that the two slots can 'see' out past some distance (basically the sensitivity of the sensor). The PIR sensor itself has two slots in it, each slot is made of a special material that is sensitive to IR. To begin explaining how a basic sensor works, we'll use this rather nice diagram PIR sensors are more complicated than many of the other sensors explained in these tutorials (like photocells, FSRs and tilt switches) because there are multiple variables that affect the sensors input and output. A great page on PIR sensors from GLOLAB \\.Parallax Datasheet on their version of the sensor.RE200B datasheet(most likely the PIR sensing element used).BIS0001 Datasheet(the decoder chip used).Power supply: 5V-12V input voltage for most modules (they have a 3.3V regulator), but 5V is ideal in case the regulator has different specs.Sensitivity range: up to 20 feet (6 meters) 110° x 70° detection range.Pulse lengths are determined by resistors and capacitors on the PCB and differ from sensor to sensor. Output: Digital pulse high (3V) when triggered (motion detected) digital low when idle (no motion detected). ![]() 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. These stats are for the PIR sensor in the Adafruit shop which is very much like the Parallax one. ![]()
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