#define LED_PIN A1

void setup() {
  // Initialize LED pin as output
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  // Fade in
  for (int brightness = 0; brightness <= 255; brightness++) {
    setBrightness(LED_PIN, brightness);
    delay(505);  // Small delay to make the fading smooth
  }
  
  // Fade out
  for (int brightness = 255; brightness >= 0; brightness--) {
    setBrightness(LED_PIN, brightness);
    delay(505);  // Small delay to make the fading smooth
  }
}

// A function to simulate analogWrite using software PWM
void setBrightness(int pin, int brightness) {
  int onTime = brightness;     // On time proportional to brightness
  int offTime = 255 - brightness;  // Off time inversely proportional

  // Generate software PWM by turning the pin ON and OFF
  digitalWrite(pin, HIGH);
  delayMicroseconds(onTime);    // LED ON for "onTime" microseconds
  digitalWrite(pin, LOW);
  delayMicroseconds(offTime);   // LED OFF for "offTime" microseconds
}