The PIR motion sensor

PIR Motion Sensor

The Passive InfraRed (PIR) Motion Sensor will, under typical conditions, detect a human being moving around within approximately thirty feet (about 10m) from the sensor. This is an average value, as the actual detection range is between 16ft/5m and 50ft/15m.

As the performance of the sensor is determined primarily by environmental conditions, no guarantees can be offered regarding the detection range. However, the range is typically sufficient for the majority of interior spaces (excluding the obviously absurd, such as basketball courts or aircraft hangars). Some garages may be large enough that one sensor cannot cover the whole area – we suggest testing this in the actual environment if there’s any doubt.

The sensor has a 110º field of view horizontally, and a slightly narrower view (approximately 70º) vertically. This means that most of what you could see if you closed one eye and put your head where you’d mount the sensor is pretty close to the area the sensor will cover. The main blind spots, of course, are above, below or behind the sensor itself.

Typically, the ideal placement in an environment is in the upper corner of a room, facing slightly towards the ground (perhaps 20-25º, depending on the height of the sensor). Try to cover typical paths through the room, focussing particularly on likely entry and exit points. If the room has several entry points, try to cover as many of these as practicable.

A visible PIR motion sensor can be a great deterrent to intruders – but on the other hand, skilled intruders can avoid obviously visible sensors. Try to mix it up a bit.

How it works:

The PIR motion sensor is a passive infrared motion sensor, meaning it detects infrared radiation rather than projecting it (unlike a security camera with infrared night vision, which is active infrared). All objects emit this “black-body radiation” (we commonly call it “heat”) and the infrared sensor looks for any of this radiation which moves.

The PIR motion sensor is not infallible – in particular, it cannot detect objects/people which are the same temperature as their background. So a human being moving about on an extremely hot day might go unnoticed. Also, a security camera with active infrared night vision in the same vicinity as the infrared sensor may give false alarms (particularly if it is a moving PTZ system).

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