the simple kind of pet resistant movement sensor uses a special lens to lift
the infra red beam above the level of the animal. fine with dogs that stay
on one level but not cats that can climb.
but the good news is there are now movement detectors (been around about
five years) that allow for one or two cats (or other animals) up to a
certain weight. They need careful siting, usually not permiting the animals
to be closer than 6 feet ie no good positioning them above a table or chair.
They do work and ive fitted a number with no trouble. they are less
sensitive than a normal sensor but do the job.

"PJO" wrote in message
...

Mortimer wrote:
I wish to install a basic alarm system to protect kitchen back door,
dining room patio doors (both at rear) and front door/hallway
downstairs and just landing upstairs which should be enough for
attack
via any bedroom. Only thing is we have two cats 15 and 17 years
old,not overly active but still walk about and have access to all the
house. What sort of alarm system should I go for, PIRs seem simple to
source and install but are there any that ignore cats and only react
to humans?

No, impossible. What "pet proof" PIR's actually do is see the area on just
one plane - horizontal from the sensor. Normal PIR's see on at least three
planes - usually 7 degrees 22 degrees and 35 degrees from horizontal. Some
have more sensitivity and include full 90 degree protection both
vertically
and horizontally.

Relying on the fact that your pet will not venture higher than the height
of
the unit itself it needs to be positioned at least the cat + tail (!)
above
any surface that the cat could get on - and that obviously includes
kitchen
worktops.

Now then... problem is that worktops are around three feet high so that
added to the cat + tail is around 5 ft. So, positioned on the wall at 5ft
high your "pet proof" PIR will probably not detect your car BUT it won't
detect anyone less than 5ft in height either and considering that many
burglars are kids you immediately have a problem.

Also consider that burglars do what they do for a living and tend to be
anything but stupid. A quick look through your window will reveal the fact
that your PIR is half way down you wall and is therefore pet proof. The
thief then simply enters the room and stays low.

In my experience pet proof detectors (sometimes called pet alley too) are
a
waste of time. They offer minimal protection and should the pet venture
any
higher than usual (up against a window for example) then you have a false
alarm.

Consider using alternative type of protection such as perimeter protection
u
sing contacts, inertias or vibration detectors. If you don't fancy that or
if it's not feasible then consider a dual tec PIR/microwave which has
sensitivity settings and pet alley on the PIR section.