Wemos D1 & Relay shield For one of my hobby projects I would like to use:


Wemos D1 mini pro (schematic)
Wemos D1 Relay Shield (schematic)


to control the 24V (19W) mist maker.

My current wiring is this:


As you can see the both grounds (12V and 5V) are attached to the com connector of the power supply. 

I wonder whether grounds should be decoupled (for whatever reason) - i.e. to use separate power supplies? If the above wiring is OK I would really like to know in which circumstances (if any) a separate power supplies would be needed.

Thanks for all your answers in advance.
That grounding scheme looks right since the Load ground is coupled back to the supply separately from the logic ground. 

The only reason you would separate supplies would be to isolate them for safety or other reasons. For example, if the load side was a large or ac voltage. 

Isolation requirements vary with application so it is hard to be more specific than that here.