int lp_led[6] = {2,4,5,18,19,21};
int i = 0;
int delaytijd = 250;

void setup() {
  Serial.begin(115200);
  // put your setup code here, to run once:
  for (int i=0;i<6;i++){
    Serial.println(i);
    pinMode(lp_led[i], OUTPUT);
    }
  
}

void loop()
  
{
      delay(10);
  // put your main code here, to run repeatedly:
  for (int i=0; i<6; i++)
  {
    digitalWrite(lp_led[i],HIGH);
    delay(delaytijd);
    digitalWrite(lp_led[i],LOW);
    delay(delaytijd);
  }
  for (int i=6; i>0; i--)
  {
    digitalWrite(lp_led[i], HIGH);
    delay(delaytijd);
    digitalWrite(lp_led[i], LOW);
    delay(delaytijd);
  }

  delay(10);

}