char Rx = 0;
int count = 0;
void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);

}

void loop() {
  if (Serial.available())
  {
    Rx = Serial.read();
    Serial.println(Rx);
    if (Rx == 'A' )
  {
    count = count + 1;
    count = count % 2;
    if (count == 1)
    digitalWrite(13, HIGH);
    else
    digitalWrite(13, LOW);
    }
  }
}