void setup()
{
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(13, OUTPUT);
  digitalWrite(13, HIGH);
}

void loop()
{
  // put your main code here, to run repeatedly:
  if (Serial.available() > 0)
  {
    char ch = Serial.read();
    if (ch == 'A' || ch == 'a') {
      digitalWrite(13, LOW);
      Serial.println("LED ON");
    }
    if (ch == '0') {
      digitalWrite(13, HIGH);
      Serial.println("LED OFF");
    }
  }
}

// S 動作時 A 也有功能
// A = ON OFF ON ....... 
// S -> on 5sec off