int ledPin = 6;  // PWM pin connected to the LED
int brightness = 51;  // Start at 20% brightness (20% of 255 = 51)
int stepSize = 26;    // 10% of 255 (round-off to closest integer)

void setup() {
  pinMode(ledPin, OUTPUT);  // Set pin as output
}

void loop() {
  // Glow up from 20% to 100% in steps of 10% with 20ms delay
  for (int i = brightness; i <= 255; i += stepSize) {
    analogWrite(ledPin, i);
    delay(20);
  }

  // Glow down from 100% to 20% in steps of 10% with 20ms delay
  for (int i = 255; i >= brightness; i -= stepSize) {
    analogWrite(ledPin, i);
    delay(20);
  }

  // Glow up again with 20ms delay per step
  for (int i = brightness; i <= 255; i += stepSize) {
    analogWrite(ledPin, i);
    delay(20);
  }

  // Glow down again with 80ms delay per step
  for (int i = 255; i >= brightness; i -= stepSize) {
    analogWrite(ledPin, i);
    delay(120);
  }
}