On Mon, 20 Apr 2015 11:41:28 +0100, Tim Watts
wrote:

On 20/04/15 11:31, 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.

MM


I would say: flush mount a reed switch in the door surround next or just
under the flap. Glue a small but powerful magnet to the flap.
You can get change over reed switches (google it). Wire it to use the
contact pair that closes when the magnet on the flap moves away from
the switch.