On Mon, 20 Apr 2015 17:01:58 +0100, MM wrote:

On 20 Apr 2015 14:06:02 GMT, David wrote:

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.

MM

Car courtesy light or fridge light?

Both operate when the door is opened.

I just checked my fridge and that has the switch pushed in by the
relatively heavy fridge door when closed and the light is off. The
switch is spring-loaded so that when the door is opened the switch
operates, closes contacts in its innards somewhere and the light comes
on.

In principle the idea is sound, but the flap wouldn't be anything like
heavy enough to depress this particular switch when the flap is closed.
A much, MUCH weaker spring would be needed. But probably doable.

MM

I refer the honourable member to my second option - courtesy light switch.

Plunger switch held away from the contact by pressure, and the contacts
closed by a spring when the door is opened.



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