Most of the times, we may not know when the postman delivers a mail or post card. People would really like to know when it arrives. It might be a nice project idea to build a notifier that would ring a bell when the mail is delivered. Also if nobody at home, the notifier should sent an Email, SMS or even a Tweet.
The Arduino Letter Box Sentry project is developed around Arduino and designed to live inside the mailbox and detect changes in the ambient light level or detect a post card passage. When the event has occurred, it triggers a wireless doorbell transmitter, ringing the house’s doorbell and letting the owner know the mail’s there. The Email, SMS and Tweeting sittings are optional and can be configured easily.