const int LED = 9;

void setup() {
    pinMode(LED, OUTPUT);
}

void loop() {
    for (int i = 0; i < 256; i = i + 1) {
        analogWrite(LED, i);
        delay(10);
    }
    for (int i = 255; i >= 0; i = i - 1) {
        analogWrite(LED, i);
        delay(10);
    }
}