Home Alarm Repair.
P.I.R.
sensors.
P.I.R.
(Passive Infra-Red) sensors are movement detectors. They have a ‘pyro’ chip
which is essentially a CCD chip (like in a digital camera) but much more
rudimentary. It makes a very pixelated image of the heat emitted or reflected
by everything in the room; The carpet gives off a certain level of heat, the
sofa another, the curtains another, the coffee table another, etc. The P.I.R.
makes a map of these heat levels and if anything changes it informs the control
panel.
Advantages;
There is no way around these. Some nutters on the internet suggest wearing a
big coat, carrying a pane of glass, throwing a burst aerosol can into the
room... Gibberish! There is no way around a P.I.R. sensor.
Disadvantages;
They detect draughts, post dropping through the letterbox, balloons, ‘floaty’
Christmas decorations and spiders (in my experience 99% of all P.I.R. false alarms
are arachnid-related. I’ve found crane-flies [Daddy long-legs], and even a big,
fat, green caterpillar inside sensors. The cable access hole is very attractive
to egg-laying creepy crawlies).
QuadP.I.R.s
.
The same as a P.I.R. but with four separate pyros. The idea is, if one pyro false-alarms the others probably won’t. All four pyros have to detect
movement before an alarm condition is initiated. In my opinion a bit of a waste
of time.
Dual-tech sensors.
Now these are a serious piece of kit. Two sensors on one P.C.B., a P.I.R. and a microwave sensor. The P.I.R. is vulnerable to all of the
aforementioned false alarms. The microwave is vulnerable to a completely
different set of false alarms. A microwave sensor works in much the same way as
the ultrasonic alarm you probably have in your car. It emits a network of
harmless microwave beams and monitors the echo, like a bat’s navigation system.
While the P.I.R. will pick up a spider on it’s lens, the microwave won’t.
Microwaves penetrate glass, concrete, brick, etc. It will pick up a cat walking
past outside. The P.I.R. doesn’t penetrate anything so it won’t. The chances of
a spider crossing the lens and a cat walking past outside at exactly the same
time are so remote they aren’t worth considering. The dual-tech is virtually
immune to false alarms. Of course they do go faulty, like all electronic
equipment. One manufacturer (I won’t name them because most of their gear is
excellent) made dual-techs which lasted around two years before failing
completely.
Some useful links;
For more detailed or specific information and advice go here.
The site is manned by experts with vast knowledge and advice which they provide absolutely free.
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