On Mon, 20 Apr 2015 12:56:27 +0100, "Brian Gaff"
wrote:

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.

No, the issue is that I need to catch the postie when he's about to
leave a card because a book or similar package from Amazon is too
large to fit through the flap. I've got TWO bell pushes, but sometimes
the dozy geezer doesn't ring either, he just taps on the door and I
don't hear it. But if he's maybe already stuffed a letter through,
then I would already have that alert to go on, rush downstairs and
open the door as he's departing whereupon he's pretty much obliged to
come back and hand me the parcel. And if he's only stuffing the
collection card through, then that would be enough to trigger the
letterbox flap alert.

I tried doing this, i.e. monitoring the postal deliveries, with VLC
and a web cam which streams the video capture to another networked PC,
but I have to be there to watch the remote display. VLC does have an
option to flash up changes from one frame to the next, but I haven't
found how to link those changes to an audible alert. Plus, it would
give all kinds of false triggers as other residents walk past my
house.

That's when I thought of the letterbox flap.


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

Yep, been giving it some more thought off and on throughout today.

MM