Each cat has a small RFID tag on the collar. When a cat is in the close proximity of the door, a small RFID reader reads the tag and if the cat is authorized, a servo will unlock the cat door. The RFID reader and the servo controller are connected to an old laptop. The software on the laptop is written in Delphi and for each "cat door event" is sending a Twitter message and a picture to twitter.com:
From the Upcoming ueue, submitted by .