So first of all, I'm pretty new to this Arduino and electronic world..
The thing is, that I want to control some components (compressor and solenoid valves, all 12V) with an Arduino and a 8-channel relay.
I found this picture for connecting:
But I'm confused, because some other pictures doesn't connect the GND from the relay to the GND on Arduino.
The Arduino should only control the Relay ON/OFF.