- Can I drive the motor from the high voltage side?
- I'm using an opto-isolator to control the DC motor. Can I ground the regulated side to the Arduino or do the sides share a common ground?
You mean the VIN on the breadboard supply? Assuming that the supply that is feeding it is adequate, yes.
Both sides of the breadboard supply share a ground.
I only need the usual protections, right--a diode to prevent back surge and so on.