Hack42, a hackerspace in Arnhem, Netherlands recently moved into some new digs, and they wanted an easy way to let their members know whether they were open or not. Fixed hours of operation typically do not fit this sort of organization, so that was out of the question. Instead, they built a switch into the wall** that will let their members know when they are open for business.
The switch separates the TX and RX pins of two Ethernet ports that reside in an old access point embedded in the wall. When the hackerspace is open, the switch is thrown and the circuit is closed. A cron job checks the state of the eth1 port once a minute, sending the “Open” status message to Twitter and IRC once it notices the status change. When the switch is thrown again and the eth1 port goes down, a “Closed” message is broadcast.
It is a simple but cool hack, and quite befitting of a hackerspace.
**No direct Google Translate link is available, though Chrome will translate it for you without issue.