OK, so I have soldered everything except the extruder electronics.
I am stuck at getting the motherboard to run: I uploaded the Sanguino firmware using the usbtinyavr, but saw no blinking. But the Arduino software claimed that everything is fine.
Now when I connect the motherboard to the power supply and press reset, I get blink-blink-blink-pause-blink. The PS does not switch on.
What do I do next?