Looking at all these options, the one I understand best is the one using a car automatic antenna. But, seeing as it is extended, which closes the door, only as long as a current is running through it, I have to wonder what happens if there is a power outage?
Then there is the add a motor, which I don't quite understand what you get with it? I absolutely love KDK1's automatically locking door that he made with it, wow! If you see this, KDK1, can you post a how to build of that door? I love it, but would want a pattern, rather than trying to make it from scratch with all the rejected trials and errors adding up, LOL