Hi,
thank you for the recommendations,
@CrossRoads, Wawa:
I read this topic:
http://forum.arduino.cc/index.php?topic=108476.0
And the issue might be related to the noise.
Why would an external pull-up resistor make a difference? (less noise presumably)
Should I then use only external pull-up resistors or add a resistor to the internal pull-up configuration?
The internal pull-up resistors should have a value between 20 and 50K so the current draw per pin when the circuit is closed should be really low (assuming 30K around 0.15 mA) and the total Arduino power drain should be well below the 800 mA.
Thanks
E.