void setup()  
{  
  pinMode(10, OUTPUT); // the declared pin must be among the PWM pins.  
}  
void loop()  
{  
  analogWrite(10, 255); // brightness increases as value increases  
  delay(1000);   
  analogWrite(10, 180);// brightness level  
  delay(1000);   
  analogWrite(10, 80);   
  delay(1000);   
  analogWrite(10, 20); // brightness decreases as value decreases  
  delay(1000);   
}