Electronic Pet Door

Hi
I'm trying to get a few ideas together to make a pet door lock.
I'm thinking something very simple that may be triggered by a small but powerful magnet attached to the pets collar, as we have a lot of strays using the door at present.
Any thoughts would be much appreciated.
Thanks

Check out these commercially available ones
Electronic, Magnetic, Infra-Red and RFID Cat and Dog Doors

This may be a bit extreme, but it's awesome!

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