Hello Yves,
Thanks a lot for your clarifications, I think I should be able to rebuild it now. I will of course first try to breadboard before I build the final shield version. A (hopefully) last question about the resistor:
What's the reason for using a 8W resistor? I thought that the transistor acts as the load and the resistor should therefore stay "cool" and does not need to have 8W. Or is it because of the precision of that types of resistors? What other kind of resistor could I use here? (it is not so easy to get, so I'm looking for alternatives)
Thanks again!