#define PIN_LED   4   //define the led pin
 
void setup() {
  pinMode(PIN_LED,OUTPUT);
}

void loop() {
  for (int i = 0; i < 255; i++) { //make light fade in
    analogWrite(PIN_LED, i);
    delay(10);
  }
  for (int i = 255; i > -1; i--) {  //make light fade out
    analogWrite(PIN_LED, i);
    delay(10);
  }
}