It would be awesome to make a pet door that responded to a specially made collar by opening. If another cat, or dog or a pest tried to get in, the door wouldn't budge. This would be nice because on hot days the dog could stay inside and not overheat, while still being able to do its buisness in the yard. And those nutsy cats would be able to go in and out as much as they pleased.
The only problem? Many cats love to shed their collars.