I used a 5th 33k[ch937]-resistor now, but it's still the same. The input values jump around and I can't read a difference when the laser is beaming on a solar cell.
I did a wiring diagramme, maybe this shows the error:
http://fabitosh.bplaced.net/arduino/schaltplaene/
Choose either the .eps or the .pdf-file.
I'd be really glad if you could help me, I got no clue.