const int txPin = 1;
const int ledPin = 13;
const char address = '0';

int ledState = LOW;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
  pinMode(txPin, INPUT);
}

void loop() {
  if (Serial.available() > 0) {
    char input = Serial.read();

    if (input == address) {

      // toggle on command
      ledState = !ledState;

      // send Ack
      pinMode(txPin, OUTPUT);
      Serial.print('a');
      pinMode(txPin, INPUT);
    }
  }

  digitalWrite(ledPin, ledState);
}