wvmarle:
What are those black things?
If MOSFETs I miss a pull-down resistor. I also miss current limiting resistors on the gate.
If BJTs of course no need for the pull-down but you really need a current limiting resistor on the base.
Those are IRLB8721 MOSFETs Logic Level, the blueprint is from Adafruit.
This is my first project, I have no experience either with transistors or MOSFETs but it works like that. I believed they know what they are doing.
AFAIK MOSFETs are voltage controlled, aren't they? Why would they need a current limiting resistor?
Regarding the pull down resistor: It is strange, when I disconnect the cable the gate can't pull down. For some reason though, it pulls down when connected to the Arduino.