They work great with the Elite electronic cat door.

This is by far the lowest cost I could find on the ID Disc but still seems to expensive.

We installed the cat door just before Labor Day. Since then our cat has lost 5 collars using these ID discs! We were lucky enough to find one of them. So be prepared to stock up on these at about $15 each if you decide to use this cat door. Our cat does have his "freedom", but you know the saying about "freedom is never free"? It certainly applies here.