Re: Choosing PIR sensors

["Mike Dalgleish" <md@xxxxxxx>]

On a simpler level, make sure, if you use these dual and more detectors, to
DISABLE the detection display.

I had a case of a burglary, where the microwave triggered, the burglar say
it, and managed to leave with his booty, without setting off the infra-red,
and hence the alarm.

In case you are wondering what was lost, it was a laptop sat next to a
slightly ajar window after painting.

Mike

> -----Original Message-----
> From: Nigel Orr [mailto:nigel.orr@xxxxxxx]
> Sent: 19 May 1999 13:55
> To: ukha_d@xxxxxxx
> Subject: [ukha_d] Re: Choosing PIR sensors
>
>
> At 12:22 19/05/99 +0100, you wrote:
> >> (which I have read elsewhere, eg alt.security.alarms) that
dual tech
> >> sensors are less reliable than standard PIRs,
> >
> >In what way?
>
> I don't know the details- it's just something I've come across in
passing.
> There's a thread in alt.security.alarms entitled 'dual tec
> detectors' where
> someone mentions some tests which found problems- maybe you could
email
> them for more details?  Andrew might have a simple technical
explanation,
> if he's reading.
>
> >We have had historical problem in work with standard PIRs and
glass break
> >detectors giving false alarms.  The new DualTech replacements
> have been much
> >more reliable.
>
> I would personally rate a piece of wet string with 2 metal
> contacts against
> glass break detectors, so that's not surprising, but maybe I've just
not
> seen any of the more reliable ones- I'm not a security installer,
> it's just
> an interesting subject to me... ;-)
>
> >> and that quad PIRs and mirror optics are both worthwhile
features.
> >
> >Can you explain these please Nigel?
>
> I can try, but hopefully someone will jump in if I get it wrong...
until
> about 24 hours ago I'd heard of the first in passing, and never heard
of
> the second...
>
> AIUI, quad PIRs (there are dual ones too), combine the signal from 4
> sensors behind the sensing lens, so can be made more sensitive to
movement
> than ambient changes.  If you didn't know (or for others who
> don't know), a
> standard PIR consists of a plastic fresnel lens with a IR-sensitive
sensor
> behind.  The lens 'breaks up' the sensor's view of the room into
zones, a
> bit like having a little array of holes so it can see some places, and
not
> others.  (actually, the little sensors in some toilets that are used
to
> control water usage just have 3 or 4 slots instead of a lens...)
>
> As you walk across the room, the sensor 'sees' heat-no heat-heat-no
heat.
> This on-off-on signal triggers the alarm contacts.  Pulse count PIRs
just
> wait for a few pulses in a given time interval before triggering,
> to try to
> avoid false triggers.
>
> The problem is that potentially false alarms could be caused by the
sun
> shining on a wall, then going behind a cloud, then shining again.  The
> sensor isn't actually sensitive to movement, just heat levels
changing.
>
> I would expect a quad sensor is a little bit more like a camera,
> so you can
> 'see' someone moving as they disappear from the view of one of the 4,
and
> appear in the view of another one.  If the sun shines on the wall,
then
> goes behind a cloud, all 4 sensors will see the same change at the
same
> time, and that can probably be ignored fairly easily.
>
> Mirror optics, I'm not exactly sure, but I would presume (from what
Andrew
> said, and looking at a pic of one of the sensors) that the plastic
fresnel
> lens mentioned above is replaced by a translucent sheet (passing IR,
> blocking visible light), with some form of multi-faceted mirror
producing
> the blocking effect, a bit like a mirrorball in a disco... the
advantage
> being that a mirror can have a sharper cut off at the edges than 5p
worth
> of plastic.
>
> Sounds plausible to me anyway ;-)
>
> Nigel
> --
> Nigel Orr                  Research Associate   O   ______
>         Underwater Acoustics Group,              o / o    \_/(
> Dept of Electrical and Electronic Engineering     (_   <   _ (
>      University of Newcastle Upon Tyne             \______/ \(
>
>
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