Implement a 4-bit binary counter

Solving Approach

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Code

// 4-Bit Binary Up/Down Counter (2026 Version)
const int ledPins[] = {2, 3, 4, 5}; // Pins for LEDs (LSB to MSB)
const int btnUp = 8;               // Increment button
const int btnDown = 9;             // Decrement button

int counter = 0;                   // Stores value 0-15
bool lastUpState = HIGH;
bool lastDownState = HIGH;

void setup() {
  for (int i = 0; i < 4; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
  // Use internal pull-ups: button reads LOW when pressed
  pinMode(btnUp, INPUT_PULLUP);
  pinMode(btnDown, INPUT_PULLUP);
  displayBinary(counter);
}

void loop() {
  bool currentUp = digitalRead(btnUp);
  bool currentDown = digitalRead(btnDown);

  // Check Up Button (State Change Detection)
  if (currentUp == LOW && lastUpState == HIGH) {
    delay(50); // Debounce
    counter = (counter + 1) % 16; 
    displayBinary(counter);
  }
  lastUpState = currentUp;

  // Check Down Button (State Change Detection)
  if (currentDown == LOW && lastDownState == HIGH) {
    delay(50); // Debounce
    counter--;
    if (counter < 0) counter = 15;
    displayBinary(counter);
  }
  lastDownState = currentDown;
}

void displayBinary(int num) {
  for (int i = 0; i < 4; i++) {
    // Check if the i-th bit of 'num' is set to 1
    digitalWrite(ledPins[i], (num >> i) & 1);
  }
}

Output

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