int led = 3;

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 <= 255; i+=5) {
    analogWrite(led, i);
    delay(2500/(255/5));
  }

  for (int i = 0; i <= 255; i+=5) {
    analogWrite(led, 255-i);
    delay(2500/(255/5));
  }
}