Instructables user gwrome has a letterbox which is quite far from his home, so needed a way to check once the mail had been delivered without walking out to check. As many people have done, he has used an Arduino and hacked existing commercial products to solve the problem. In this case an extra button for an existing wireless doorbell system was modified to be triggered by an Arduino board.
A light sensor is connected to the Arduino and detects the change in light when the letterbox is opened - which triggers the doorbell, which triggers the user to walk over and check the mail. Hopefully it isn't junk!
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