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:
The Arduino should only control the Relay ON/OFF.
thx
Please give us a link to the actual relay card you are going to use.
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Hi,
Details, diagrams about this subject HERE
NEXT thing is Solenoids etc. Can be ElectroMagnetic Interference territory.
Some help HERE
The relay I'm going to use: https://www.aliexpress.com/store/product/Optocoupler-8-channel-5v-relay-module-shield-board-for-arduino-control-panel-5V-8-channel-relay/113294_32693315899.html
But I bought mine from a Danish website, not Aliexpress 
The picture shows a 5V supply and I guess I'm gonna need a 12V supply?
If the relay coils are 12vdc, you will need an external 12 volt supply.
If they are 5vdc you will need an external 5 volt supply.
What is written on your relays?
SRD ? ? ?
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On the relay It's written 10A 250 VAC, 10A 125VAC and 10A 30VDC, 10A 28VDC.. But since all my components (compressor etc) is 12V DC, I guess im gonna need at 12V supply?
"On the relay It's written 10A 250 VAC, 10A 125VAC and 10A 30VDC, 10A 28VDC.. "
That sounds more like the load the contacts can switch. 10A at the different AC & DC voltages.
What is being asked has to do with the voltage needed across the coil to energize the coil and switch the contacts.
Can be 5V, 12V, 24 - which is it?
Yes, as Crossroads says.
What is written after:
SRD
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Oh sorry guys, It says SRD-05VDC-SL-C
Those are 5 volt coil relays.
They take 5 VDC to energize the relay, 'NOT' 12VDC.
You will need an external 5 VDC supply.
Do not use the Arduino 5 volts.
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So I need a 5V power Supply for the relay and a 12V power supply for the components?
