Yes, they are really cheap and the law (here in Sweden at least) requires all outdoor cats to be chipped. So the cat is probably already chipped anyway.
Even if for some reason you didn’t want to chip your cat (you should absolutely chip your cats and dogs) it would be trivial to just put a tiny receiver in something that dangles off their collar for the door to communicate with.
Do those exist? Does seem like an obvious option.
Yes, they are really cheap and the law (here in Sweden at least) requires all outdoor cats to be chipped. So the cat is probably already chipped anyway.
Even if for some reason you didn’t want to chip your cat (you should absolutely chip your cats and dogs) it would be trivial to just put a tiny receiver in something that dangles off their collar for the door to communicate with.
They have existed about as long as RFID has been available.
I had one about 10 years ago