#define LED1 13
int incomingByte = 0;
void setup()
{
  Serial.begin(9600);
  pinMode(LED1,OUTPUT);
}
void loop(){
  if (Serial.available() > 0) {
    incomingByte = Serial.read();
    if(incomingByte =='a')
    {
      digitalWrite(LED1,HIGH);
    }
    if(incomingByte =='b')
    {
      digitalWrite(LED1,LOW);
    }
  }
}