void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(13,OUTPUT);
  digitalWrite(13,LOW);
}

void loop() {
  // put your main code here, to run repeatedly:
    if(Serial.available()>0)
  {
    char data =Serial.read();
    if(data=='a')
    {
      digitalWrite(13,HIGH);
    }
    else if(data=='b')
    {
      digitalWrite(13,LOW);
    }
  }
  else{
    digitalWrite(13,LOW);
  }
}