🛠 Lire un bouton poussoir avec résistance de pull-up interne

Objectives

Learn how to read a digital input on the Mega 2560 using the internal pull-up resistor (20 to 50 kΩ according to official documentation), avoid mechanical bouncing with debouncing logic, and turn on an LED when the button is pressed.

Hardware

• 1 × Arduino Mega 2560 + USB cable
• 1 × push button
• 1 × LED + 1 × 330 Ω resistor
• Breadboard and wires

The button is connected between pin 2 and ground (GND), without an external resistor: the logic is inverted (LOW = pressed) because the internal pull-up keeps the pin at 5 V when idle. The Mega 2560 has 54 digital pins, so the pins with external interrupts (2, 3, 18, 19, 20, 21) remain available for more advanced projects.

Complete code

// Bouton + pull-up interne — Arduino Mega 2560
const uint8_t BTN_PIN = 2;
const uint8_t LED_PIN = 13;

uint8_t etatPrecedent = HIGH;
uint32_t dernierRebond = 0;
bool etatStable = false;          // false = relâché

void setup {
  pinMode(BTN_PIN, INPUT_PULLUP); // pull-up interne 20-50 kΩ
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(115200);
}

void loop {
  uint8_t lecture = digitalRead(BTN_PIN);
  uint32_t maintenant = millis;
  if (lecture != etatPrecedent) {
    dernierRebond = maintenant;   // rebond détecté
  }
  if (maintenant - dernierRebond > 30) {
    bool etat = (lecture == LOW); // logique inversée
    if (etat != etatStable) {
      etatStable = etat;
      Serial.println(etat ? F("bouton pressé") : F("bouton relâché"));
    }
  }
  etatPrecedent = lecture;
  digitalWrite(LED_PIN, etatStable ? HIGH : LOW);
}

Expected verification

Open the serial monitor at 115200 bauds: each press displays "button pressed" only once (no bursts due to bouncing) and the LED follows the state of the button. When idle, the LED is off.

Common errors

• Forgetting INPUT_PULLUP and leaving the pin as a floating input: the state changes on its own, the LED flickers.
• Forgetting the logic inversion: with a pull-up, the pressed button gives LOW, not HIGH.
• Debouncing time too short (less than 20 ms): multiple ghost presses for each press.
• Using the "button to 5 V" wiring with the internal pull-up: short circuit from 5 V to ground when pressing.