Proximity sensor for dog door ?

[Sam]

I'm planning to make a PIC controlled automatic dog door for my Bernese Mountain Dog puppy, I've looked at the commercially available doors and they have many reviews indicating that they don't last long and cost $$$.

The only problem I have is how to detect when only my dog needs to go in or out. I know I need to have some type of transponder or transmitter on his collar, either RF or IR.

My car has the keyless entry/start system and I know of retrofit kits for this and I could hang the "key" off his collar and feed the input to the PIC.

I'd appreciate if anyone could please pass along any ideas or suggestions on how to do this.

Thanks for any help !