Hello vffgaston,
Thanks for the help on the ground. It is clear know. Since I am using the same power I guess it will not be necessary.
However the signal is sent FROM the raspberry pi TO the arduino... And thats one of the problems. Raspberry pi HIGH is 3.3V and arduino HIGH is 5V (more or less, actually is between 3 and 5 but we cannot be sure that the raspberry PI will send a 3.3 V exactly).
I guess my question should be posted on a raspberry forum...
Alexis