Read a logic state.

The challenge

Light the LED while the button is pressed and count each press only once. A mechanical contact bounces: one action can produce several electrical transitions.

Equipment: the previous LED setup and a normally open button. With four pins, use a multimeter to identify the already-connected pairs; the drawing represents two distinct contacts.

Wiring and logic

The internal pull-up holds the input high when the contact is open. Pressing connects the input to ground: the logic is active low. No external pull-up resistor is required for this lesson.

The complete program

// D2 -> bouton -> GND. LED : D8 -> 1 kohm -> anode ; cathode -> GND.
constexpr uint8_t BOUTON = 2;
constexpr uint8_t LED = 8;
constexpr unsigned long ANTIREBOND_MS = 30;
int lecturePrecedente = HIGH;
int etatStable = HIGH;
unsigned long dernierRebond = 0;
unsigned long appuis = 0;

void setup() {
  pinMode(BOUTON, INPUT_PULLUP); // Resistance interne : relache = HIGH.
  pinMode(LED, OUTPUT);
  digitalWrite(LED, LOW);
  Serial.begin(115200); // Aucun while(!Serial) : fonctionne sans ordinateur.
}

void loop() {
  const unsigned long maintenant = millis();
  const int lecture = digitalRead(BOUTON);
  if (lecture != lecturePrecedente) {
    dernierRebond = maintenant;
    lecturePrecedente = lecture;
  }
  if (maintenant - dernierRebond >= ANTIREBOND_MS && lecture != etatStable) {
    etatStable = lecture;
    if (etatStable == LOW) { // Front d'appui : un seul comptage.
      ++appuis;
      Serial.print("Appuis : ");
      Serial.println(appuis);
    }
  }
  digitalWrite(LED, etatStable == LOW ? HIGH : LOW);
}

The debounce filter

lecturePrecedente stores the raw reading. Each change restarts the timer. A reading unchanged for 30 ms becomes etatStable. The counter increases only on a stable transition to LOW; holding the button does not create a new press.

30 ms is an educational setting to verify. Too short allows bounce through; too long makes the system less responsive.