void setup() {
  //configure PWM controller congfiguration.
  ledcSetup(0, 5000, 8);
  //declare the GPIO number for PWM signal output.
  ledcAttachPin(2, 0); // ledcAttachPin(GPIO,channel)
}
void loop() {
  //function for() to create decremental value by 1 start from 255-->0
  //from OFF LED to linear increasing brightness, for active-low circuit.

  for (int brightness = 0; brightness < 255; brightness++) {
    //ledcWrite() function will generate PWM output signal according to variable brightness value
    //1st argument:PWM channel number. //2nd argument:Tone frequency.
    ledcWrite(0, brightness);
    delay(15);
  }

  for (int brightness = 255; brightness > 0; brightness--) {
    //ledcWrite() function will generate PWM output signal according to variable brightness value
    //1st argument:PWM channel number. //2nd argument:Tone frequency.
    ledcWrite(0, brightness);
    delay(15);
  }
  // delay(1000); //wait for 0.2 seconds before start again.
}