Thanks to a busy life we can often forget simple things - such as if an outside garag door is closed, or be waiting for the mail and not want to walk out and check the box every half an hour. Instructables member BrianH who uses two simple sensors which are monitored with an Arduino-compatible circuit to keep track of outside events.
A light sensor is fitted inside his letterbox to detect the mail, and a reed switch and magnet is used to monitor the status of the garage door. The neat thing is that the system will beep once every ten minutes as a polite reminder to close the garage door, or you can reset or otherwise control this through the code. And with a little effort it can be fitted in a neat enclosure that wouldn't look too much out of place:
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