int d = 10;
void setup() 
{
  pinMode(3, OUTPUT);
}

void loop() 
{
  for(int a = 0;a<=255;a++)
  {
    analogWrite(3,a);
    delay(d);
  }
    for(int a = 0255;a>=0;a--)
  {
    analogWrite(3,a);
    delay(d);
  }
}