Re: Choosing PIR sensors

[Nigel Orr <nigel.orr@xxxxxxx>]

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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