Simple Read

Read data from the serial port and turn ON the onboard LED if the character received is an 'a' For the Wiring boards v1 the on-board LED is on pin 48, on Wiring S the on-board LED is on pin 15.

int data; // to read the char

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

void loop()
{
  if (Serial.available())       // if data available
  {
    data = Serial.read();       // read data
    if (data == 'a')            // if value read is character 'a'
    {
      digitalWrite(WLED, HIGH); // turn ON the onboard LED
    }
    else
    {
      digitalWrite(WLED, LOW);  // if not turn it OFF
    }
  }
  delay(100); // wait 100ms for next read
}