A few years ago I bought an Reloop RP3000 direct drive turntable. On the back it has a remote control port. This port protocol is undocumented and no one mentions it on the web.
Mono 6.3 MM Plug
So I had to take a look how it works. After much searching on the web how it could work I found that a MONO 6.3 mm plug should be inserted with a switch on the back. Then, I had to find out what the polarity was. I added the switch and it works to start / stop the turntable. There is a small quirk however, that if you press the switch sometimes you have to press it again to start over. Maybe a flick switch was better, but I’m very glad with the result as I can now start it from a remote distance.