const int ledPins[] = {2,4,5,16,17,18,19,21}; // GPIO pins connected to LEDs
const int numLeds = 8;
int count = 255; // Start count at 255 (all LEDs on)

void setup() {
  // Set all LED pins as OUTPUT
  for (int i = 0; i < numLeds; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop() {
  // Display the current count on the LEDs
  for (int i = 0; i < numLeds; i++) {
    digitalWrite(ledPins[i], (count >> i) & 1);
  }
  
  // Delay for a second
  delay(1000);
  
  // Decrement the count
  count--;
  
  // Reset the count if it goes below 0
  if (count < 0) {
    count = 255;
  }
}