"MM" wrote in message
...
In my rummage box I found a 20-year-old (at least) Polycell door
alarm! White in colour, this measures about 125mm x 33mm x 25mm. It
runs on a 9V battery, which, believe it or not STILL had enough power
to emit a few feeble clicks from the buzzer when I switched it on and
played around with the keeper.

Yes, there's a keeper to go on the door; the alarm is screwed to the
door frame. I never used it. It was a flash in the pan idea when I
lived near High Wycombe which had a high crime rate.

Now, I've dismantled the thing! The circuit board inside (approx 65mm
x 30mm) was fixed with two plastic "rivets" which I drilled out in
seconds.

With a new 9V battery the alarm is nice and loud as I remember it.

Without the casing gubbins around it, this is quite small and compact
enough to mount on the letterbox flap. But the 64,000 dollar question
is, how long could I extend the two wires leading to the buzzer?

The buzzer looks like a small round black drum, approx 12mm dia and
about 15mm long. It has 2 wires coming out of it. It is marked P87J.

How about 3 metres? 10? I'd use bell cable or similar. Would extending
it by such a length eventually "blow" the components on the circuit
board (extra stress pumping the signal that much further)?

Should be fine for any distance you like in a house.