Is the issue that stuff is left in the opening making it stay open, or does
it always snap back every time. I ask as it would have a bearing on the
design, ie, latching or non latching.

What would I do? Small hall effect IC, and a relay with a magnet on the
flap.

These only have three leads like a transistor and detect the magnet either
being there or not, so really its a case of where you glue your magnet or
you could just use the hall effect device to operate a changover relay, or
you could use it to just bias on or off a transistor if you don't want to
use a relay.

BY the way, many of the cheaper door phones use a very handy way to make the
door bell sound. They wire the outside speaker/micropphone though a
capacitor, and then, shorting this out triggers the noise, so if you
already have a door phone, one could design a basic simple microswitch on
the flap so as it is pushed out, it shorts the capacitor and sets off the
door bell.
Of cours this might cause confusion as both the door bell and the flap would
sound the door bell!
Brian

--
From the Sofa of Brian Gaff Reply address is active
"MM" wrote in message
...
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