There's a saying that can often sum up the relationship between cats and humans - "Dogs have masters, but cats have staff". If you would like to turn the tables on your cat(s) - consider this RFID-controlled cat (or other pet) door. Exercise control over the feline movements at your discretion - and ban those who used to sneak in for a free meal. By simply mounting the RFID and Arduino hardware described by Instructables user "landmanr", and two solenoids to control the flap - you're set. With the addition of a real-time clock you could also add time-based access to the system. You can see the solenoids and system in action below:
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