thanks CrossRoads, but i guess this would change also the whole wiring, right?
It works actually fine, the only issue I'm having now is that the Serial thing messes with my idea of sending an ascii character from Processing in order to trigger one specific solenoid. And I guess this wouldn't be too hard, but since the whole thing already starts acting strangly when opening the Serial.available() in arduino I'm asking myself if there are other solutions to communicate with processing or to use serial communication without influencing the arduino/register thing...