#define CLK 2
#define DT 3
#define LED_PIN 9

int brightness = 128;
int lastStateCLK;

void setup() {
  pinMode(CLK, INPUT);
  pinMode(DT, INPUT);
  pinMode(LED_PIN, OUTPUT);

  lastStateCLK = digitalRead(CLK);
}

void loop() {
  int currentStateCLK = digitalRead(CLK);

  if (currentStateCLK != lastStateCLK) {
    if (digitalRead(DT) != currentStateCLK) {
      brightness += 10;
    } else {
      brightness -= 10;
    }

    brightness = constrain(brightness, 0, 255);

    analogWrite(LED_PIN, brightness);
  }

  lastStateCLK = currentStateCLK;
}