alexisgaziello:
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).
Yes, you're right. Although may be the arduino sees 3.3 V as logical "1", I wouldn't risk: use a level shifter; they are unexpensive.
Regards.