The Ani-Mate company produces electronic pet doors for cats and small dogs. These doors run on a 9 volt battery. The small size has a 5 ½ inch by 5 ½ inch opening and is suitable for average cats. The larger 7 inch by 7 ½ inch size for dogs up to 15 lbs and large cats.
The Lexan flap seals to the frame with a pile weather strip, and is held shut with a magnet. In addition to the electronic lock there is a manual 4 way lock that can be set for in only, out only, fully locked, or fully open.
These doors are normally used to keep strays out, but can be installed "backwards" so that the door will let only your pets that have collar keys out. When doing this type of installation, it is important to caulk the battery cover to keep water out of the electronics.