On 20/04/2015 12:50, wrote:
On Monday, 20 April 2015 12:32:28 UTC+1, Bod wrote:
On 20/04/2015 12:21, Rod Speed wrote:


"Bod" wrote in message
...
On 20/04/2015 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

What about fitting a tiny tilt switch, maybe in series with a doorbell?
A very simple solution, or is that a silly idea.

Not that easy to have a connection to it that will last for decades.

Ok, what about a reed switch or a magnetic switch in unison with a
separate buzzer/bell? Or don't you think that will last?

No, you'd need to feed the reed switch output to a transistor, use that to drive the bell. Reeds have very low ampacity.

Use a hall effect transistor and you can dispense with the reed switch.
You then have a "switch" with no moving parts that will last "forever".


--
Cheers,

John.

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