int LED = 9;
int brightness = 127;
int delaytime = 500;

// brightness = 0   => duty cycle = 0%
// brightness = 64  => duty cycle = 25%
// brightness = 127 => duty cycle = 50%
// brightness = 255 => duty cycle = 100%

void setup()
{
  pinMode(LED, OUTPUT);
}

void loop()
{
  brightness = 255;
  analogWrite(LED, brightness);
  delay(delaytime);

  brightness = 127;
  analogWrite(LED, brightness);
  delay(delaytime);

  brightness = 64;
  analogWrite(LED, brightness);
  delay(delaytime);

  brightness = 0;
  analogWrite(LED, brightness);
  delay(delaytime);
}