#define LED 8
void setup() {
  // put your setup code here, to run once:
pinMode(LED, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
for(int i=0;i<20;i++){
{digitalWrite(LED,HIGH);
delay(20-i);
digitalWrite(LED, LOW);
delay(i);}
delay(10);
}

for(int i=20;i>0;i--){
{digitalWrite(LED,HIGH);
delay(20-i);
digitalWrite(LED, LOW);
delay(i);}
delay(10);
}
}