PWM - Pulse Width Modulation

Duty cycle: 500Hz corresponding 2ms (Milliseconds) in time.

void setup() {
  for(int i=17; i>=2; i--){
    pinMode(i, OUTPUT);
    digitalWrite(i, HIGH);
  }
}

void loop() {
  for(int i=2; i<=17; i++){
    digitalWrite(i, LOW);
    delay(100);
  }
  for(int i=17; i>=2; i--){
    digitalWrite(i, HIGH);
    delay(100);
  }
}