I would like to attach a sensor to the bottom of my letterbox so that when a parcel or a letter is dropped in it I would receive instant notification. Ideally the sensor would be able to be configured to certain impact threshold and to report the value.

What kind of sensor could I use for that purpose? What kind of device could it be connected to which would be able to trigger a network event (e.g. HTTP request)?

The letterbox is this one (not my photo):

enter image description here

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    \$\begingroup\$ I think it would be better to use something to detect when there is a letter in the mailbox, so that it's not possible to miss it when it happens, so that you'll detect even small letters, and so you won't detect trucks driving past. \$\endgroup\$ – immibis Aug 1 '18 at 5:45
  • \$\begingroup\$ Something like a photocell? If the sensor goes dark, message delivered \$\endgroup\$ – Alexander von Wernherr Aug 1 '18 at 5:47
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    \$\begingroup\$ @Greendrake: Then you may have to add a platform and loadcell (a weighing scales) in the bottom of your postbox. With that connected to a microcontroller you can detect changes in weight whether the item lands with high or slow velocity. \$\endgroup\$ – Transistor Aug 1 '18 at 6:03
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    \$\begingroup\$ Why not a simple switch on the shutter? \$\endgroup\$ – Maple Aug 1 '18 at 8:50
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    \$\begingroup\$ I was thinking if your mailbox has a lid or flapping shutter then you can add a switch to it. Whenever something placed inside the switch closes and lights up a LED. The whole thing is just a few parts and about an hour of work, including attaching switch to mailbox. The simpler the design the more reliable it is. \$\endgroup\$ – Maple Aug 1 '18 at 10:42

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