Alarm question - PIRs, window contacts or optical boundary?

Personally I'd forget window contacts. They do false alarm if there's a gust of wind or a bird hits one. The actual units (Vipers etc) are prone to damp. The wiring is very labour intensive unless it's done at first fix stage in a new building. It is possible to make it invisible but it's hard work.

PIRs are generally OK as long as they are wired (not wireless) and are dual tech (microwave as well as IR).

I have external beams to protect the parking area. They're pretty good for that purpose, but not for protecting the house when you aren't there because of perfectly innocent callers (unless they send a silent alarm to your phone and/or run the CCTV). The type with two independent parallel beams are best.

With all alarms remember that one false alarm means that everyone within earshot will ignore your alarm for the next ten years.

Bill

What about fog? Brian

I was thinking of using sealed reed switches (which is what I used in the previous house) - how do they get affected by damp?

The existing PIRs are wired, but aren't "animal proof" so I need to either replace them or change to switches. The problem is still what happens when people wander about in the night when the alarm is set.

It is a lot of work for not much gain. Burglars tend to go for empty houses anyway and the PIR trigger will go off soon enough to deter them. My dad had external perimeter reed switches fairly neatly done but it was a lot of work and wiring to be hidden. I just have window locks.

They don't but they do sometimes end up stuck closed so that they don't actually trigger by opening when they should. The panic alarm button on my system is a magnet and reed switch and failed this way.

You mask out all the zones that you are likely to walk through for the night setting (and/or remember to switch it off when you get up).

Good point (snow too) but the problem can be minimised by choice of power (I'm aware of the laser regs) and distance, and the gate passage detectors seem to work OK in practice. There's an interesting article here:

You can get dual-beam detectors, which are supposedly spaced so that animals/birds/trees in the wind etc will only break one beam, but humans will break both and set them off, then use corner reflectors to bounce the beam round your perimeter up to 100m, they use pulsed beams presumably for anti-tamper.

This has the same answer as the old joke about the man who goes to his Doctor and says "It hurts when I do this".

Don't do that, then.

None of them are.

:-) I would like to control what some of my guests do in the night but I might get into trouble if I tried

Thanks, I'll look around. It reminds me of that old heist film where the actress (supposedly) had to learn to gymnastically gyrate around a room full of IR beams - Entrapment, I think.

Some claim to only respond to the thermal image of an object over 30kg - that's a pretty large dog or moggy!

We have one of those in the garage.

A bat set it off. We had it "adjusted". A bat set it off.

Bats have the ability to get closer to the PIR than a dog down on the ground, presumably they're quite warm when not roosting?

Newfoundland.

Bats tend to fly up into corners of rooms (looking for something they can grab hold of), and that's exactly where PIR's are often mounted.

Maybe there's circuitry within the sensor (assuming it's not actually a combined PIR/ultrasonic one) which happens to oscillate in the bat's hearing range that they find "interesting" ...

Window switches: Enamelled copper wire can be disappeared in a superficial scratch in plaster. Make sure to fit them to part of the frame where distor tion & wind can't trigger them. Check the current is in range for the reed relay, they tend to fail if it's too high or too low. They don't of course detect broken glass. You can get glass break detectors too.

PIR: they will sense you if you go there. Full stop.

Beam break: used that long ago, very effective. Can sometimes be placed whe re you won't walk at night, and where any door shutting will trigger it. Ma ke sure the detector & mirror can't ever slip out of alignment or it'll tri gger.

Personally I'd want to treat one detection event as a minor warning, and 2 as a person present rather than 1.

NT

Window frames are terrible for condensation. It gets into connections.

If these people close an internal door it can set off a window contact.

Bill

It isn't possible.

Bill