Bonsoir,
dans la référence http://arduino.cc/en/Tutorial/DigitalPins il y a une note concernant la pin 13 en entrée:
(désolé, mais je ne préfère pas traduire .....)
NOTE: Digital pin 13 is harder to use as a digital input than the other digital pins because it has an LED and resistor attached to it that's soldered to the board on most boards. If you enable its internal 20k pull-up resistor, it will hang at around 1.7 V instead of the expected 5V because the onboard LED and series resistor pull the voltage level down, meaning it always returns LOW. If you must use pin 13 as a digital input, use an external pull down resistor.
peut te donner des indices pour avancer. Si j'ai bien compris il faut ajouter une résistance de tirage au 0v"