int ledPins[] = {2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}; 
void setup() { for (int i = 0; i < 12; i++) {
   pinMode(ledPins[i], OUTPUT);} 
}
void loop() { for (int i = 0; i < 12; i++) { 
    for (int brightness = 0; brightness <= 255; brightness++) {
      analogWrite(ledPins[i], brightness); delay(500); 
    }
    for (int brightness = 255; brightness >= 0; brightness--) {
      analogWrite(ledPins[i], brightness); delay(500); 
      }
    }
  }