Thanks for your replies!
I tested it and it actually works (Ok only 3 Leds because they are brighter then. Thx, retrolefty)! Hurray!
But! I would really like to undrstand why?! Where does the GND from the Arduino board itself come from??
Is the GND some sort of reference for the transistor? I thought a transistor is like a switch, triggered by the
base so that the circuit from +12V to GND is closed? Why do I need the GND from the Arduino?
Where does the GND from the Arduino come from when I would like to build a standalone ATMega328 circuit?
I designed a little circuit (hope I made nothing wrong with the standalone ATMega wiring):
OMG... Im such a noob... please forgive me!
Thanks in advance!!