Where not to put a motion sensor

When I’m in people’s homes talking about potential alarm systems, they always ask me where the sensible place to put a motion sensor is.
What I’m actually going to do here is to tell you where a bad place to put a motion sensor is.

Now first of all I need to explain how a motion sensor works. They’re called PIR sensors, that stands for passive infrared and they work by sensing the movement of heat. We, as human beings, have body heat as do all types of wearm-blooded animals, and what basically happens is that the sensor divides the room up into a grid and when it sees heat in a particular cell in the grid and it moves from one cell to another, then that counts as movement of heat.

Now obviously that’s quite a simplistic way of explaining it, the algorithms built into the senses are a lot more complicated than that, but it’s a good place to start off with.

So the first place that I’m going to tell you not to place a movement sensor is directly opposite a heater or radiator. Now obviously those heat the room up, hot air will rise and that’s movement of heat.

The second place that you shouldn’t really put a motion sensor is directly facing a window, for a similar reason. When it’s sunny the sun comes streaming in through the window and you know what it’s like, it heats up the air just inside, that air will rise and that again is movement of heat.

Now often it can be quite difficult to put a sensor in a room where it’s not facing a window or a radiator, however modern motion sensors have built into their algorithms patterns which guard against that potential false alarm. WIth older systems, it probably it can be a problem.

The last place that I’m going to suggest that we don’t put a motion sensor is directly facing the stairs and that’s if you’ve got a cat or a dog that will roam the house. Most modern motion sensors are sensitive to smaller bodies of heat, cats, dogs etc, but very often when they’re at eye level, directly at the same level as the motion sensor, it can trip them. We found that out to our cost at one of the first systems that we ever installed, the people’s cat was walking down the stairs and setting off the system. We moved the sensor away from the stairs and put it elsewhere in the hall and the false alarms stopped.

So those are the three places that are a bad idea to put a motion sensor.