Shed alarm going off a lot - heat?

I have a PIR alarm fitted in my shed to try to stop the local tea leaves but the bloody thing has been going off at night (around about 12pm). Checking inside all I could find was a monster spider which I dismissed... but it went off again after that.

Does anyone know if the heat built up during the day could cause convesction of anything that might cause sufficient movement to set it off?

if it goes off at the same time it must be something that happens at that time.

With regard to spiders etc, have you taken your pir apart, they may be some hiding inside or behind the pir.

also is the shed "secure" i.e, no windows or vents left open?

If you cannot pinpoint any potential problems, then try replacing it with a dual tech (infrared/microwave) or a quad.

I had a similar problem in a commercial coldstore (I know, other end of temp. scale!), but a dualtech stopped the system falsing all the time.

try changing the bedtime of the spiders so that they are awake during the day and go to bed at 10

large spiders in particular can still set off a detector, it may also be vermin.


you really want to find the cause, not the problem

buy an alarm with a contact rather than a PIR

the advantage of a detector is tat it covers a whole area, a contact on covers one point, so that would be a waste of time for a shed (wouldn't go off if they came intruder window (now you know why they are called intruder alarms)

Buy the spider a proper spider bed so that he doesnt see the PIR as his best option.

When you have alot of heat present like this weekend spaces like sheds can have a large amount of heat build up. Once that happens the PIR can be set off for a number of reasons, moving air currents some warmer than others, spiders moving inside the casing having a magnifying effect on the PIR, sun or sudden shadows though a window.

As mentioned above, a dual tech is the safe option for this environment as the microwave is not affected by temperature but be careful to set the detector up properly otherwise the microwave may go through the shed perimeter and detect things outside.

Cheers people.

The shed is very new and we have two cats that tend to check it out regularly so vermin is unlikely... I will check the device to see if it has become a spider home though... as we do seem to be spider central for some reason.

I do have 8' of windows on one side of the shed that the alarm probably catches in its periphery, but movements outside should not set it off surely... it doesn't when we walk past anyhoo... and shadows at the time of night....

This does make me wonder what the technology is behind these things... I have a science background but passive infra red does imply heat to me rather than movement... but then I do profess ignorance on this technology in my defence

The PIR monitors the existing infra-red conditions and reacts to a sudden change in them. So if it was a human being there would be a sudden increase in infra red registered as we are a infra-red source. If there are inra-red levels inside the shed, like a heater, or a car if it were a garage, moving air currents can be seen by the PIR as a sufficient change in infra-red levels. Spiders inside the lens can fool the PIR in to thinking the infra-red level change is much larger than it actually is.

I take your point if it is happening at night then the window idea does not seem likely. With the outside temperature levels being so high recently it could be moving air currents setting it off or insects/spiders, I don't know.

Have you checked the pulse count on the PIR? This should be set to 2 normally and you can set it to 3 to make the PIR slightly less sensitive. (Pulse count- The number of times that the PIR needs to be triggered before going open circuit)

oooo that sounds like a good idea. I sort of suspected there may be convection currents whirling around in there all invisible like... and I suppose a breeze through a loose fitting room or summat could change them all quite a lot. I will check for the pulse count though I do not recall seeing that option... it was £25 so not the cheapest on the market but then again... not exactly top end.

Thanks for the help again.

a pir actually looks for the movment of infra red heat. It uses two pyroelectric sensors mounted "top to tail" this prevents false alarms the very first pirs had

then there is a choice of 2 things depending on brand

1) the sensor looks at a mirror which is "segmented" (bit like me putting you in your car, bolting your head still, and putting several rear view mirrors infront of you so you can only see behind you)
then if you see something move from one mirror to the other you "go off"

2) instead of mirrors the sensor has a fresnel lens (search google) this also allows the sensor to see various places


useless but true

most have a white "window" to screen out white light

some can see 50m away

some think there is a beam you cross, this missconception is brought about by the makers supplying a drawing that shows lines where the pir can see, people then interpret these as beams, its not its where the pir can see (remember the rear view mirrors?)


there is also a whole bunch of electronics to "filter out" spurious activations

some have pulse count which means the number of times it must see something before it goes off

you can not alter the range with a dial, you can lower it down where its fixed (this also means they should never be too high)

you can also tilt them forward when mounting it.

they do not like sudden draughts

sunlight (even refleted of a shimny surface)

they can not see through solid objects (except polythene)
hot or cold air

cable entry holes should be sealed as this prevent wind turbulance inside the detedtor (also venturi effect)

spiders set them off by obscuring its vision, then when they move the pir can now see something that wasnt there before

like most things you get what you pay for, cheap ones are usually prone to false alarms

breezer said:

bit like me putting you in your car, bolting your head still, and putting several rear view mirrors infront of you so you can only see behind you

Click to expand...

Didn't know you were into this kind of practice, Breezer??