Reliably detecting letters in a letterbox

What’s the best way of detecting letters or small bits of paper in a letterbox like the one shown in the pictures attached?
There’s a slot at the front for small letters and the front is hinged to allow for parcels that are too big to pass through the slot. The top is designed for holding newspapers (which I don’t get) so I may block off the front to make a secondary storage box.

For the bottom half I was thinking of a grid of 6 IR leds on the lid facing downwards towards a corresponding grid of IR sensors on the floor of the mail box.

This has to survive Australian summer conditions where the temperature regularly reaches 60+ deg C (140F) in the sun. I can bury the majority of the electronics underground where the temperature is less extreme.

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Images from Original Post so we don’t have to download them. See Image Guide





What about a flap with a light spring that has to be pushed back to insert an object - the flap could be connected to a micro-switch. No electronics needed.


The LED system might work but you can't guarantee the letters (or whatever) will land flat - they might be wedged in funny positions, or curled up if they are flimsy leaflets.

So to adapt your idea I would put some kind of detector (or two, spaced out) at the delivery slot - an LED shining on an LDR might be enough so that any brief change in light intensity received means someone has put something in that slot, no matter how small or flimsy. IR LEDs would work I guess too (they too are affected by sunlight though). Perhaps you could modulate them to a nice frequency, 32Khz, that would be much more sunlight resistant.

You would also have to re-baseline the light received by the LDRs / IRs every hour or so as the sunlight may affect them, but that's just a simple line of code.

Still thinkin' ::)

  1. IR led and IR photodiode to detect anythong placed in the box through the slot.

  2. A sprung flap with a reed switch.

  3. Maybe even a PIR to detect someone within x meters of the box (assuming postman or local post thief).