🛠 Allumer une LED avec un bouton poussoir

Objectives

Read a digital input and control an output: the button/LED pair, the foundation of any interaction. You will discover internal pull-up resistors (INPUT_PULLUP), the active-low configuration, and the phenomenon of mechanical bounce.

Materials

• 1 Arduino UNO R3 + USB-B cable
• 1 breadboard, connecting wires
• 1 push button
• 1 LED + 1 330 Ω resistor (to limit current below the 20 mA per pin ceiling)
• Wiring: button between pin 2 and GND (no external resistor needed, the internal pull-up does the job); LED + 330 Ω between pin 13 and GND.

Complete code

// Bouton sur pin 2 (vers GND), LED sur pin 13.
const uint8_t BTN = 2;
const uint8_t LED = 13;

void setup {
  pinMode(BTN, INPUT_PULLUP);  // pull-up interne : repos = HIGH, appuyé = LOW
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
}

void loop {
  bool appuye = (digitalRead(BTN) == LOW);  // logique inversée (actif-bas)
  digitalWrite(LED, appuye ? HIGH : LOW);
  Serial.println(appuye ? "appuye" : "repos");
  delay(10);
}

Expected verification

LED off at rest, on when pressed. The Serial Monitor (9600 baud) displays the state continuously. If you remove the wire from pin 2 without the pull-up, the input floats and the display becomes random — this is precisely the behavior that INPUT_PULLUP corrects.

Common errors

• Forgetting INPUT_PULLUP and wiring the button to +5 V without a pull-down resistor: floating input, erratic readings.
• Inverting the logic: with this setup, pressed == LOW. Many beginners test for == HIGH and think the button is broken.
• Forgetting the LED resistor: the LED burns out or the pin saturates (max 20 mA).
• Mechanical bounce causes the reading to jitter: invisible here, critical as soon as you count presses — see the multitasking millis tutorial with 30 ms debounce.
• Wrong serial port: the UNO resets every time the port is opened (the bootloader intercepts the first bytes); the first characters may be missing.