Trigger for a postbox monitor

(Not sure if this is the right category to use, feel free to move)

I’ve just ordered myself a Hologram Dash to build a basic postbox monitor - that is, something which can sit in my physical mailbox downstairs and notify me when someone puts something inside it. The idea, silly as it might be, is to avoid having to go downstairs 3 or 4 times each morning to check for post.

Alas, the postbox is outside the range of my WiFi, otherwise I’d have probably gone for the Adafruit Feather Huzzah, but I welcome this opportunity to play with GSM bits and pieces.

Anyway, right now that’s less of a concern - what’s more important right now is figuring out what sort of switch or trigger I can use to recognise that something has been posted.

One approach I considered is to use a PIR sensor, which I’m hoping would recognise movement, but I’m worried this might get damaged if anything large is posted, and feel there must be a better approach, which is why I’m posting here.

I’ve attached some photos of the postbox - I figure there might be some type of switch which I can attach to the flap, but I’m not very well versed in such things, so would welcome some suggestions.

What you could do is use an infra-red beam detector, similar to devices you may have seen being sold as children’s “laser trip wire” toys.
The basic idea is, an infra-red beam is shone from one unit onto another, which has a photo-detector on it. If the beam breaks, then you receive a signal which you can use to notify yourself.
You could attach one of the modules to the inside of your postbox’s flap, and the other one further down inside the box. When the postie comes to open the flap, the beam will be broken and whallah! “You’ve got mail!”

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I’d look at a reed switch with magnet on the flap or a tilt switch fixed to the inside of the flap.
Not sure where in the world you are, but this should give an idea of the sort of thing I’m suggesting


That’s a fun idea, but unless I’m looking at the wrong things, it seems a tad too big. I might save such an idea for putting a (joke) alarm on my office (to stop the girlfriend from getting in ;))

I like this idea, thank you for the link. I’m in the UK, and there’s a Maplin not too far from here.

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Instead of the kids toy use this, with what ever microcontroller you fancy. This will tell you if the box is empty or not. and no need to reset the alarm. Once you remove your mail the beam makes contact again.

The read switch and magnet is likelier easier to wire up and get working. But will only tell you if the door has been opened. You’ll have to latch the alarm state. As soon as the door is closed again the read switch will close again. Then reset the alarm when you remove your mail.

Things that come to mind, are you allowed to modify this mail box? Asking because you’ll need power if you put a microcontroller in the box. Battery powered would work but that means recharging it or swapping the battery every so often. Looks like a metal box so WIFI isn’t going to work very well, going to run a wire from it to your apartment? If your mail carrier see’s wires going in he or she may refuse to even touch the box.

Oohh, I like this idea.

What positioning was you thinking about inside the box? Maybe one on the back wall, and another on the door, down near the bottom? I could maybe 3D print a little ramp for it, so incoming mail won’t end up sitting on top of either of them.

No, probably not, so any solution I come up with is going to need to be removed when we move out. So the use of hot glue, or some other type of temporary adhesive, will be used to fix things in place.

I was planning on running it from a battery, with a process which would inform me when the battery was low. I’ve not worked with batteries before, so I’m interested to see how long they’ll last, etc…

The idea is that everything will be self contained within the unit, without any external features showing. I plan on using the Hologram Dash to run it all, using it’s GSM features to send signals.

Of course would be a good one to buy 😀


Thank you for your suggestion. I was considering a tilt sensor (and think I have one laying around somewhere), but was worried that it would require the flap to be all the way opened to register - whereas I’ve got a feeling that it’s probably only half-way opened most of the time.

An alternative I’ve thought of using is a vibration sensor switch - this doesn’t need the flap to be fully opened, as partial opening (and the movement it generates) should be enough.

You could also use a small accelerometer like the one available on the EviropHat. I know from experience that it’s really quite accurate, and would detect any movement in the flap at all.

You could even use a sole Zero W attached to the flap in order to get your readings, without too much bulkiness, so long as the cellular data device fits. :D

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After giving this some thought, one problem with the breaking the beam approach, is a letter laying flat on the bottom of the box. It will be so thin it won’t break the beam no matter how low you place the sensors. Unless you detect a break and save that state. Same deal as the read switch. Keep your indication lit until you get your mail and reset the alarm state. Another issue is, either the door could be opened or just the flap used depending on the size of the item being put in the box. Your going to need to detect both.

I use a powerboost 1000c with one of my portable Pi projects. It has a low battery LED and a low battery output pin on the board you can monitor. I haven’t bothered to make use of it though. I have a 6600 MAH battery that I don’t see ever running down completely. I only ever run it for a couple of hours tops on battery. And I’m monitoring it while its on.
If your going to swap batteries and charge them in your apartment you won’t need the 1000c, just a regular powerboost 1000 or 500. Plus a LIPO charger board and a second battery.

Before I do anything, I think I’d check to see if you can send any RF signal out of the box. If you can’t send data out no point in going any further, IMHO.

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Because it’s using a GSM connection, which is limited to 1MB of data per month, I’m less concerned with the idea of instant status of the mailbox, and instead was working on the idea of a simple switch, which in turn triggers an outbound message.

It’ll probably trigger a second time when I open the mailbox to get the post - although as you suggested before, maybe I can have a reset switch (or even something as ambient as a light sensor, which detects the mailbox being opened, and uses this to reset the status.

I’ve got a feeling I’m going to try a bunch of these different options, and see which works the best.

Thank you for your battery suggestion.

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That’s the way I’d go, only send a message if the box is opened. And only send the one message until its reset. Then when you retrieve your mail. Press the reset. I’d also add a delay to the reset to give you time to close the door etc.


You could not glue the switch in a ‘part tilted’ position, say 30 deg off vertical, so that it would trigger before the flap fully opened. Don’t know how sensitive the vibration sensors are but is there a risk of false triggers if opening an adjacent box rattles yours?
Theres a link from the pimoroni tilt switch to some debouncing code that might help regardless of which sensor you use.


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You have to be careful as to how the postal worker fills the boxes. Usually there is one master key hole that opens the whole front cover/door. They don’t open each one separately, they just open the whole front of the unit. That’s how it was when I last lived in an apartment. Attach something to the small door and it could get ripped off when the large main door is open. If I’m right there should be one out of place keyhole down one side of the main box…

That may well be the case with some other mailbox configurations, but this one simply lets the postal worker flip the flap and pop in the post.

Ok, it was just a heads up, in case it had a master key etc. Mine had the slot too, but that was for everybody else to use. Like the I was here but you didn’t answer delivery guy notices etc.

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