On Mon, 20 Apr 2015 11:31:51 +0100, MM wrote:

I want to hear an alarm sound/see an LED flash when the postie posts
something through my letterbox. The letterbox has a flap on the inside
to keep draughts down. I want to attach 2 contacts to the flap and
body such that as soon as the flap opens and the contacts are broken,
said alarm/LED are triggered.

I'm worse than a novice in electronics, although I've painstakingly
soldered Velleman kits and similar before. I don't know the first
thing about circuit design, but I know enough to know what a resistor
is and what the coloured rings represent. Also, capacitors,
transistors and, above all, DIL chips such as the CMOS 4000 series.

I've reviewed several circuits on the internet, but they all seem
overkill for what I need. The problem, it seems, is getting the thing
to trigger when the circuit is OPENED.

Thanks.

I should have though that the simplest would be a standard battery
bell circuit with a microswitch on a springy piece of wire behind the
flap replacing the bell push. Adjust the microswitch position so that
it is far enough behind the flap not to be set off by gusts of wind
but close enough that any post passing through the flap will trigger
it. The springy piece of wire should be springy enough to allow the
flap to open fully and the microswitch to return to the waiting
position each time the flap closes.

It won't look terribly elegant.

Nick