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

[MM]

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

[Tim Watts]

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.

[dennis]

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.

[tabbypurr]

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

[Bod]

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

[Nick Odell]

I should have though that the simplest would be a standard battery bell circuit with a microswitch on a springy piece of wire behind the flap replacing the bell push. Adjust the microswitch position so that it is far enough behind the flap not to be set off by gusts of wind but close enough that any post passing through the flap will trigger it. The springy piece of wire should be springy enough to allow the flap to open fully and the microswitch to return to the waiting position each time the flap closes.

It won't look terribly elegant.

Nick

[Davidm]

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.

[Brian Gaff]

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

[Chris J Dixon]

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

[David]

Car courtesy light or fridge light?

Both operate when the door is opened.

[MM]

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

[MM]

That's not a bad idea, actually. Dunno where I'd find such a piece of wire, though.

Not unduly worried about that.

MM

[MM]

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.

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

MM

[MM]

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

[dennis]

You need a PIR connected to a chime, then it detects him walking up and not pushing anything through the flap.

You can buy them as cordless battery operated.

[David]

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.

[Andy Burns]

I'd say use an alarm contact with its magnet as your trigger, e.g.

Or put a post-it note on the door asking the postie to ring the doorbell whenever they deliver something :-)

[newshound]

I think you mean in parallel (with the bell push). Still readily available (Maplin, Farnell) but alas you can no longer get the reliable, high capacity mercury ones from RS which the IRA used to use.

[spuorgelgoog]

This apparently will switch a buzzer from a 9v battery

A possible source might be the pendulum tilt switches used for heater safety cut-offs eg

Float switches using microswitches and weights have high current capacity but lack finess in operating angles.

Mercury switches still appear to be available; this supplier mentions ratings of associated connection leads up to 40 amp

Owain

[Nick Odell]

I was thinking of something that worked along the lines of an ironing board flex holder. Not necessarily this one:

._SY355_.jpg

but something that is both stiff enough to hold the switch in place and bendy enough to fold out of the way when the flap is opened.

Nick