Thanks for taking the time to draw it out Bluejets, that makes it much clearer. I see how the optical sensor controls the relay starts or stops the motor depending upon the position it's in. I take it the relay an incoming connection then 2 (switched) outputs. Actually, I'm thinking of using an Arduino to control everything (with shields?) because I need to start it remotely, via the Internet - so a virtual start button if you will. The big problem with that is learning enough programming code as necessary to do it. Still, should be fun learning! Do you know if I'd need an Ethernet shield or a WiFi shield?
Colin, I'm a complete newbie to your world, so I have to say I don't know what "registrations" is, sorry. But I'm leaning quickly as I go, so would be grateful if you expand a little. I wouldn't want to assemble a device around a motor only to find it's not suitable.
Relay I showed was a double pole if that's what you are referring to.
As far as controlling with the Arduino, a relay can be driven from an Arduino output via a transistor or a small mosfet quite simply.
Output of it would then replace the "start button". There are other ways but this is simple.
The programming for that part would be straightforward being an output that would come on for maybe 1/2 second or so each time it was required to move the disc.
The wifi or ethernet is just another section of the code required.
Once you do some Arduino coding you will see that most is relatively easy and there is plenty of help on the Arduino forum.
Just they prefer you to try at least to write your code and then help is readily available.
Remember to start one thing at a time and once each bit works, try to combine them.
Good luck with it and remember to walk before you run.