PIR positioning

[R D S]

I installed an alarm at work, only got around to doing downstairs. I placed the PIRs such that they weren't opposite windows, one though can catch glints of light depending on which way the vertical blinds are overlapping. After a handful of false alarms over the years I would swear that alarms only happen on sunny evenings when the blind slats are such that light can hit the sensor. I wouldn't have thought that this should be a problem, but that is my experience.

A friend who used to run an alarm company recently fitted an alarm upstairs. He place the PIRs opposite windows and I have had numerous false alarms on one in particular, the one facing a window where the sun can be seen setting, i've ended up covering it up. No bother since.

What are other folk's experiences and opinions of PIRs and their positioning, cos my mate reckons light shouldn't affect them, and i'm inclined to agree that it shouldn't through a DG window. So maybe it's just coincidence?

[Chris Hogg]

PIR = Passive Infra Red . The clue is in the words. Sunlight contains infra-red radiation. Lots of it. It's what makes you feel warm when it shines on you. Infra-red radiation just below the visible spectrum will pass through ordinary window glass. It's why greenhouses and conservatories get hot in summer. If sunlight falls on a PIR detector through a window, the detector will register it and trigger. The only way a PIR detector won't trigger it is if the level of infra-red radiation changes only slowly. But if a cloud passes across of the sun, the infra-red radiation will switch off and then back on again quickly, and the detector will think someone has walked in front of it and trigger.

[Andrew Gabriel]

Sunlight probably won't although I'd avoid positioning one so it is exposed directly, but heat caused by sunlight can make things like blinds move around in convected air currents.

When I first installed mine, I had an issue where the landing sensor tripped about 10 mins after the heating came on. This was due to hot air rising from the downstairs hall radiator making the upstairs light pendant move around. Replaced the pendant flex with a solid rod, and the problem went away.

My usual advice when installing an alarm yourself is don't connect up the external sounder until you've managed a couple of months without any false alarms. Otherwise your beginner mistakes will have used up any good will your neighbours might have had by the time your alarm is working properly, and they won't bother investigating when a real incident happens.

[Martin Brown]

Sun hitting a part of the sensor but not all of it will possibly generate a false trigger condition. It is in essence an edge detector so uniform illumination shouldn't trigger it.

Although it is true that PIR is passive infrared they are optimised to detect human body temperature longwave thermal IR and window glass is more or less opaque at those wavelengths. The problem likely stems from something warmed by the sun like window blinds moving slightly due to convection from rising warm air.

It isn't a good idea to put PIRs where direct sunlight can hit them or where they may be facing airconditioners or heaters capable of creating fast changing local thermal gradients. Most of my false alarms have been bats flying round the room or spiders inside the detector.

All of my PIRs face into the room and away from sunny windows.

[AlanC]

I have sensors in several rooms facing windows (north and south) and one is also above a radiator.

I read all of the recommendations about not doing it but they were really t he only locations that I could place them without a lot of hassle.

The one facing the patio door (which is south facing) gave a few false alar ms so I disabled it and left it like that for a few years until I spotted d ual-tech sensors which PIR and micro wave. I replaced the existing sensors with these and haven't had any false alarms since. You can set them up so they only trigger when PIR and micro wave are triggered (which is what I h ave done) or either.

Alan