int led_pin = 3; //define variable for the led pin
int led = 0;

// the setup routine runs once when you press reset:
void setup() {
  // declare pin 9 to be an output:
  pinMode(led_pin, OUTPUT); //set the led_pin as output
  pinMode(led,OUTPUT);

  Serial.begin(9600);
}

// the loop routine runs over and over again forever:
void loop() {
  
  for (int i = 0; i <= 250; i++) {
   
    analogWrite(led_pin, i);
    
    analogWrite(led, 255-i);
    
   delay(10);

  }

}