int led=9;          // PWM pin led is attached to is 9
int brightness=0;   // how bright the light is
int fadeAmount=5;   // how many oints to fade led by

void setup() {
  // put your setup code here, to run once:
  // here pin 9 given above=LED is set to be an output 
  pinMode(led, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  //  digitalWrite(ledPin, HIGH); used then can't change brightness so written analog
  analogWrite(led,brightness); // set brightness to pin 9

  brightness=brightness+fadeAmount; //change brightness for next time in loop

  if(brightness<=0 || brightness>=255){
    fadeAmount= -fadeAmount;
  }

  delay(10); //wait 10ms to see dimming effect
}
// Anode (+, long leg) → Pin 9 of Arduino (through a 330Ω resistor)
// Cathode (-, short leg) → GND on Arduino