What is simplest possible circuit for letterbox flap "open" detector?

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
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On 20/04/15 11:31, MM wrote:

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.
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On 20/04/2015 11:41, Tim Watts wrote:

Just buy an alarm door contact or even one of those cheap battery powered window alarms they had in poundland, self adhesive, contain a reed switch and a sounder. I think they were a pound for four, I bought some because I wanted the batteries for something else.
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wrote:

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.
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Davidm wrote:

That could provide a signal of the right sense.
Does the OP need a circuit which latches, for the rare occasion when the postie doesn't leave the mail stuck in the flap, with a gale howling through the gap? ;-)
Chris
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Chris J Dixon Nottingham UK
snipped-for-privacy@cdixon.me.uk
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wrote:

Well, that doesn't happen, but it would be nice to have the alarm alert for 10 seconds before switching off. For starters, it's enough to just have the gadget sound once. For just a letter, it wouldn't sound for very long, but I usually get either no mail or a bundle of up to four items and it takes longer to push those through.
MM
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On Monday, 20 April 2015 11:31:54 UTC+1, MM wrote:

Simplest is a microswitch. But the flap needs to move away from it to trigger, could glue a piece to the rear of the flap very close to the hinge to create that
NT
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On 20/04/2015 11:31, MM wrote:

What about fitting a tiny tilt switch, maybe in series with a doorbell? A very simple solution, or is that a silly idea.
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Not that easy to have a connection to it that will last for decades.
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On 20/04/2015 12:21, Rod Speed wrote:

> 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?
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On Monday, 20 April 2015 12:32:28 UTC+1, Bod wrote:

No, you'd need to feed the reed switch output to a transistor, use that to drive the bell. Reeds have very low ampacity.
NT
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On Monday, 20 April 2015 12:50:55 UTC+1, snipped-for-privacy@gmail.com wrote:

you could use a lever micro switch such as this http://www.rapidonline.com/design-technology/cherry-dc1c-a1ld-microswitch-spdt-6a-250v-ac-long-lever-solder-ip67-50-2600

depends on the bell I'd use a buzzer and this switch takes up to 0.5amps http://www.rapidonline.com/design-technology/aluminium-body-reed-switch-78-1002
have to make sure the right type with regard to normally open or normally closed.

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On 20/04/2015 12:50, snipped-for-privacy@gmail.com wrote:

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".
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Yeah, that's a much better approach because the switch and the wiring to it can be on the fixed part of the door and its trivial to make it completely invisible too if you want.

Yeah, that will last fine.
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On 20/04/2015 12:58, Rod Speed wrote:

> Righto.
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In my rummage box I found a 20-year-old (at least) Polycell door alarm! White in colour, this measures about 125mm x 33mm x 25mm. It runs on a 9V battery, which, believe it or not STILL had enough power to emit a few feeble clicks from the buzzer when I switched it on and played around with the keeper.
Yes, there's a keeper to go on the door; the alarm is screwed to the door frame. I never used it. It was a flash in the pan idea when I lived near High Wycombe which had a high crime rate.
Now, I've dismantled the thing! The circuit board inside (approx 65mm x 30mm) was fixed with two plastic "rivets" which I drilled out in seconds.
With a new 9V battery the alarm is nice and loud as I remember it.
Without the casing gubbins around it, this is quite small and compact enough to mount on the letterbox flap. But the 64,000 dollar question is, how long could I extend the two wires leading to the buzzer?
The buzzer looks like a small round black drum, approx 12mm dia and about 15mm long. It has 2 wires coming out of it. It is marked P87J.
How about 3 metres? 10? I'd use bell cable or similar. Would extending it by such a length eventually "blow" the components on the circuit board (extra stress pumping the signal that much further)?
MM
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On 20/04/2015 19:07, MM wrote:

Have you tried a little card next to the letterbox:
"Please ring the bell after you put anything through the letter box. Thanks."
That would be effective, and "simplest possible".
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wrote:

Each bell push has a laser printed label (had so for months) saying "Please ring BOTH bells!" You have to realise that not everyone can read nowadays. They can recognise house numbers and somehow they work out which street or road they're on, and that's about as far as their academic experience ever went. My postie (well, one of them; I get several different ones) can't even tell the difference between 7 and 5, because sometimes I get the neigbour's mail and she gets mine.
MM
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Yeah, plenty don't bother to read stuff like that.

The posty always can because he has to read the letter/parcel label.

They can only do that by reading the street signs initially.

Sure, but no one who gets to be a posty leaves school now without being able to read.

I get some mail delivered to the same street number on the next street over which has a MUCH longer street name. I know that because the old fella who lives there comes over and knocks on the door and we have a bit of a chat when he does that.
The posty clearly just puts it in the wrong bin when sorting the mail before putting it into the bag on the motorbike.
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On Monday, 20 April 2015 19:07:18 UTC+1, MM wrote:

Any reasonable distance if the buzzer is the sort with internal driver circuitry and the alarm is only sending DC volts down the wire.
Not sure if it's a bare piezo sounder relying on a driver circuit on the main alarm board.
Owain
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