I have an alarm system that has a PIR in the detached garage. Every time we
go on holiday the alarm triggers via the garage PIR. I can pinpoint it to
when the dustbin men come by looking at the alarm log, and also my neighbour
tells me! I leave a key with her so she can turn off the alarm and reset.
Now my (limited) understanding of PIR's is they work on heat/movement. How
is it being triggered by the bin lorry? The lorry only gets as close as
approx 50 feet, so I do not think vibration is the cause. My thoughts are
the flashing lights on top of the cab. There is a gap at the top of my
garage door of approx 3/4"and the PIR is moulted at approx this height, is
it possible that the 'lighthouse effect' of the lorry light is somehow
crossing the PIR sensor and triggering the alarm or am I barking up the
I have had a guy look at it (not an alarm engineer but an electrician
acquaintance of a friend) and he seems to think it will be down to spiders
although he couldn't see any evidence of webs etc in or around the PIR, also
the spider theory wouldn't explain why it only goes off on a Weds morning
when the bin truck arrives, unless of course the spider is hiding from the
On Thu, 26 Feb 2009 10:04:12 -0000 someone who may be "John"
Sounds unlikely, unless the flashing lights are emitting a fair
amount of heat. However, the lorry has a large engine which does
give out rather more heat.
Is the sensor "looking" out through this gap? If so events outside
may trigger it, depending on the detection pattern.
Sensors should not "look" directly at windows or gaps, to minimise
false alarms. If your sensors do then they were installed
incorrectly. Note that this is not the only factor in placing such
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
I believe it could be down to spiders inside the passive. Check inside the
device for evidence and make sure that the cable entry is sealed to keep
them out in future.
Also consider getting a dual-tec passive for this location - far less prone
to these false alarms.
Thanks for this but if it is down to spiders etc. why does it only trigger
when the bin men arrive (anytime between 7.15 and 8.15 AM). Also there
doesn't appear to be any evidence of spiders webs etc and the cable entry
hole is blocked with a dab of silicone sealant. I have some damp proof
membrane that I can use to cover the gap on the inside of the garage door to
see it this helps.
Is this the same as my understandinfg of a 'double knock' zone?
OK forget the spiders.
I had occasional false alarms from a hall passive and replaced with
dual-tech which has not given any problems.
Haven't heard the expression "double knock" but there is a description here
No; 'double knock' means the sensor has to trigger twice to alarm.
Some PIR sensors have a jumper to select such behaviour. Alternatively,
some alarms can be programmed to look for double knock (and sometimes
across different zones).
Dual tech is two different technology sensors in one unit, normally
PIR and radio wave doppler shift. The units are normally jumpered to
only generate an alarm if both detectors trigger. They are less
liable to false alarms, and are used in more expensive commercial
alarm installations, but there's no reason you can't use them in
a home alarm. (They are sometimes used in garages where hot air from
a recently parked car can trigger a PIR, and in conservatories where
heat from the sun can generate false PIR alarms.) They require careful
setup to ensure the microwave isn't seeing outside the desired
protection area. Many of these have anti-masking detection, which will
either trigger the alarm contacts or the tamper contacts if the doppler
detector picks up movement which the PIR doesn't see, so you don't
want the doppler detection zone spilling outside your garage doors,
for example. They are typically twice the price of a good PIR (or
perhaps four times the price of a dirt cheap PIR).
[email address is not usable -- followup in the newsgroup]
No, in all probability right idea, wrong part of lorry. It's most
probably the exhaust, which on many lorries has a vertical section to
chuck it out at height. Another possibility is that compressing the
waste of many households will cause the rubbish itself to rise in
temperature - I'm not sure by how much, but it happens to hay when
it is baled - and thus the whole lorry is warm compared with its
I have a PIR that activates an alarm at my front door, to warn of
visitors and prowlers, and I think it is sometimes set of by my
neighbour's heating outlet, which is about 15m away, perhaps because
of the clouds of steam and exhaust that it causes to swirl around in
front of the house. It also seems to be sensitive to voltage spikes.
When I was doing up a cottage, in the lean-to that contituted the
temporary toilet, there was a spider's web under the basin, right in
front of where one sat on the toilet. One day, I found that if I sang
(vocal range approximately baritone, if anyone else wants to try :-)
when on the toilet , the vibrations in the web would bring the spider
charging out of hiding, thinking it had caught something.
I don't know whether that is what is happening, as I said the lorry only
gets as close as 50 feet. The PIR doesn't pick me up on my driveway when
cleaning the car etc and I am only 20 feet away. I realise trhat I am
smaller than a bin lorry but is it really likely that the heat from the
lorry (or engine/exhaust) is being picked up through a 3/4" gap?
I am going to try this but if the gap wasn't there the door wouldn't open,
as the door is 2" thick and sat in a 2" thick wooden frame. I appreciate
the gap may be a little on the large side for clearance but all the other
garage doors on my Cul-De-Sac are the same, and sometimes if not opened
correctly the door fouls on the back edge of the frame.
They work on movement of heat sources, so you on your driveway may be
too faint and or occupy too small a solid angle against the background
heat 'noise' in the environment, especially if you have a dark
coloured car, whereas the side of the bin lorry will occupy a much
bigger solid angle than you, and its exhaust will be much hotter than
As you suggest, try covering up the gap for the duration of a few bin
men visits, and if that fixes it, consider moving the unit.
I doubt spiders have alarm clocks. Why don't you just try angling the
sensor downwards so that it won't be triggered by movement of a warm
object at 50 ft? 30ft should be plenty, you can test the activation
range of course by doing a walk-by in broad daylight if you increase
the ambient light sensitivity control first.
To be honest, I'd never considered putting a PIR in a garage as there just
seemed too much possibility of drafts and spiders triggering them in error.
What I did at my last house was run an infra-red dual beam detector inside
the garage and covering the entire length of it on the side with the
window/side door. So anyone breaking through the widow (or getting past the
reed on the main or side doors for that matter) would end up breaking the
beams. Both beams (a few inches apart) had to be broken so even a large
garden spider crawling across the detector wouldn't trigger it.
Google "dual beam"+detector to get an idea what sort of products are
available. They are more commonly used for outbuildings and perimeter
Note this was on a wired system, but I think some manufacturers do wireless
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