int led = 13; 
int data; 
void setup() 
{ 
  Serial.begin(9600); 
  pinMode(13, OUTPUT); 
} 
void loop() 
{ 
  while(Serial.available() > 0) 
  { 
    data = Serial.read(); 
    Serial.println(data); 
    if(data == 'A') 
    { 
      digitalWrite(13, HIGH); 
    } 
    if(data == 'B') 
    { 
      digitalWrite(13, LOW); 
    } 
  } 
}