About to install a yale wireless alarm system. it ststes in the instructions not to point the PIRS at a window. In 2 of my rooms this would be how I would want to locate them with both windows being approx
24 feet from the PIR. I can if necessary do them in reverse. Is this likely to cause an issue? What is the issue with pointing at a window as I dont understand how PIRs actually work.ss was thinking very hard :
Looking at the window, they would be able to also see any movement outside the window, so even if there were no vehicles or pedestrian movement, an animal passing by or a window cleaner doing your windows could trigger the system. If you must point them towards a window, maybe worth seeing if you could direct them down so the window is not visible to the PIR, but without excluding any internal motion.
Basically PIR's detect IR heat from bodies and rapid changes in IR temperatures. You can blank off areas of the lens if necessary, with black electrical tape.
They look for sharp warm to cold transitions moving around. You can sneak up on most PIR sensors if you move really slowly.
More likely because rapid thermal changes from sunlight and clouds could cause a false trigger. Window glass is virtually opaque to thermal band IR so it isn't a worry about external things moving.
Ours get false triggered by spiders dancing around inside them (cure is a film of vaseline on the case) and bats getting into the rooms.
Better not to point it directly at a window. North facing would be OK.
In article , Harry Bloomfield writes
Not true, glass is long wave IR opaque.
However sunlight can have heating effects on the glass leading to hot air currents that the PIR can interpret as a moving body, triggering the sensor/alarm. Same basic reason that they should not point at radiators.
All modern sensors will have pulse count technology[1] which should dramatically reduce such false alarms.
The o/p wants to aim the sensor so that an intruder would walk across the front of the detector (at whatever distance) whilst avoiding it pointing at any nearby changeable heat source (radiator, window, area of carpet subject to strong sunlight. Remember it is passing through zones that results in a successful trigger.
[1] There are basically 2 sense zones copied multiple times across the sense area. Processing circuit triggers only when an IR emitter passes through multiple times (on-off-on-off).In message , ss writes
Probably too late now but microwave/PIR detectors are less trouble as they need to see a moving object as well as a moving heat source. Curtains etc. blowing in a draft can trigger the microwave section but are not hot so don't trip the PIR and hence the alarm. Heat from the sun or a radiator will be seen by the PIR but as it is not physical will not trip the microwave.
They work by rate of change on adjacent areas of infra red sensitive material. The problem seems to be that they are pretty crude and anything that can vary in temperature will often set them off. I don't know if it sees the scene outside the window, or whether the air dropping near a window does it. I had some issues with a heater as well. Of course turning the sensitivity down is often the way to go.
I have not played with them for some years now, so maybe they behave better these days.
Brian
In message , Martin Brown wrote
And radiators fitted beneath windows causing localised heat changes..
Wanna bet? ;-) Moving curtains certainly can trip a dual-tech detector.
Also, microwave detection without IR detection can result in an alarm or fault condition in a dual-tech detector with anti-masking protection.
Microwaves can "see" through windows (and some other building fabrics), so it's important to set the microwave detection range accurately.
Thanks all for the replies, I now have a better understanding of how to site the PIRS. I should be able to eliminate any potential issues.
You cant blank off lens areas with much useful effect, due to the way the optics work.
NT
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