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When planning motion sensor placement for a security system, is it preferable to install it facing windows, or not?

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4 Answers

up vote 7 down vote accepted

Presuming you're talking about passive infrared or PIR detectors, you do not want them pointing at your windows. They detect changes in the ambient temperature in the field of view, and pointing at windows causes them several problems:

  • heating registers are often located under windows, so when your heating system turns on and off, the temperature will change rapidly just there.
  • if you have changeable weather, there can be big temperature swings as clouds block and unblock the sun.
  • drafts from windows can make drapes sway, blocking and unblocking part of the window.
  • the lenses in PIRs are designed to admit infrared light and block visible light, but they're not perfect. The more ambient light in the field of view, the less sensitive they'll be.
  • windows can often be a magnet for pets, and while pet-immune sensors exist, they often rely on blocking off the part of the PIR window that looks downwards. If your cats are walking on the back of the sofa or your dog is jumping up and down barking at squirrels in your yard, it brings them directly into the field of view.

Think about what an intruder would be taking and how they'd get out of the house (it's easier to go out the back door than a window carrying the big screen TV), then cover the areas that they'll be going through.

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+1 for looking at the same wikipedia page as I was :) –  BMitch Aug 28 '12 at 19:32
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IR motion sensors can be thought of as a low quality camera with only one color, IR. They are constantly comparing the IR levels across a grid to see when there is a quick change in IR levels. When you look at the housing of a sensor, you can see this grid, and unlike that of a digital camera with mega-pixels, these sensors only have a dozen or so measurement points.

Motion directly towards or away from a sensor will show only a small change in IR at the same point. Therefore, you typically want the sensor position to detect motion from side to side. Locate them in places you need higher levels of security, like your home office, or in places a burglar would have to pass, like a stairway or central hallway. You also want them positioned that a pet could not accidentally trigger it, either by aiming high or by placing them in parts of the home that would be closed off.

Also, since these sensors are detecting a change in heat, avoid pointing them towards HVAC vents or other sources of quick temperature change. And avoid pointing them towards windows since IR can pass through glass (though various coating on newer windows should reduce this problem).

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detect motion side to side is a key point for placement. –  Niall C. Aug 28 '12 at 19:42
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The short answer is no, you need to position the motion sensors in a manner where they cover open areas like hallways. You might have windows here or you might not. If facing away from the window gives you a larger coverage area then that is probably a better placement.

The windows should be separately secured with contact sensors and optionally glass break detectors.

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You want to point it at the area that you predict an intruder would be most likely to move/travel. Windows and doorways would be among the prime locations for that.

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