gistfile1.ino

void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);
}

char data;
int f = 0;
void loop() {
  if (Serial.available() > 0) {
    data = Serial.read();
    if (f) {
      digitalWrite(13, HIGH);
      f = 0;
    }
    else {
      digitalWrite(13, LOW);
      f = 1;
    }
  }
}