by Jan Buiting @ elektormagazine.com:
Even in the Internet & Wi-Fi age where it seems irrelevant just where you are at a specific time, it’s sometime useful to know if a visitor called at your door while you were out. Who knows, it might be the DHL man with a delivery of essential components!
The little circuit described in this article can reassure you that you haven’t missed the postman’s call irrespective of whether he always rings once or twice. It’s cheap and can probably be built from parts in your junkbox.
Doorbell Memory – [Link]