How to isolate low voltage control circuit from high voltage power circuit?
That's exactly the application optocouplers are made for.

Optocouplers consist of a LED on one side and a photodiode or a phototransistor on the other side. Both are put into one package but are kept well insulated from each other.

The electrical signal is transformed into light by the LED and back into an electrical signal by the photodiode/phototransistor.