#include <SoftwareSerial.h>

#define RX    1   // *** PB1, MISO
#define TX    0   // *** PB0, MOSI

const int ledPin =  4; 

char c;

SoftwareSerial mSerial(RX, TX);

void setup() {
  // ***
  // *** Initialize the Serial port
  // ***
  mSerial.begin(9600);
  mSerial.println("Initializing...");
 
  pinMode(ledPin, OUTPUT);
}

void loop() {
  if (mSerial.available())
    c = mSerial.read();
  if (c == 'a'){
    digitalWrite(ledPin, HIGH);
    //delay(1000);
  }
  else
    digitalWrite(ledPin, LOW);
}