Paul__B:
That circuit should work as far as I can see, but you could do just as well by arranging one Arduino pin to be continuously generating a square wave output and driving the "pump" with the two capacitors and two diodes.Since the current requirement is so low and you can "pump" at a high speed, you could use 1N914 or similar diodes and 1µF capacitors.
Thx for the information but my question is still upp. Why the negativ voltage dropping in the circuit? I drive it with a transformator powered 12V 1A standard power supply. I dont want to use en arduino pin for this purpose.