Wawa:
Non-opto isolated relay boards with 24volt relays (if they exist) must have ground as common for all the inputs.
Not common VCC, as opto isolated boards.
Leo..
Thank you Leo,
So just to sum it up, and in order to learn the working principles, let's say such module exists, for the discussion, and inputs have ground as common (which means arduino digital pins acts as ACTIVE HIGH):
There is no problem to have a common ground to the 24V power supply and to the arduino, correct?
And the 75mA or so, for driving the coil of the relay, will be drawn from the power supply and not from the arduino, is that correct as well?