include <WProgram.h>
  
  int ledPin = 13;   // select the pin for the LED
  int val = 0;       // variable to store the data from the serial port
  
  void setup() {
    pinMode(ledPin,OUTPUT);   // declare the LED's pin as output
    Serial.begin(9600);        // connect to the serial port
  }
  
  void loop () {
    // read the serial port
    val = Serial.read();
  
    // if the input is '-1' then there is no data
    // at the input, otherwise check out if it is 'a'
    // if it is, send back a message "ABC"
    // and turn on and off the LED on pin 13
    
    
    if (val != -1) {
      if (val == 'a') {
        Serial.print("ABC");        // send back a message simple as ABC
        digitalWrite(ledPin, HIGH);
        delay(200);
        digitalWrite(ledPin, LOW);
      }
    }
  }