#include <SoftwareSerial.h>

SoftwareSerial mySerial(0,1); //(3, 4); // RX, TX

#define LED_PIN 7 // 5
bool led = false;
#define BTN_PIN 3 // 9
bool prev_btn = false;
bool btn = false;

void setup() 
{
  // Set the data rate for the SoftwareSerial port
  mySerial.begin(4800);
  pinMode(LED_PIN, OUTPUT);
  pinMode(BTN_PIN, INPUT_PULLUP);
  digitalWrite(LED_PIN, led);
}

void loop() 
{ 
  btn = !digitalRead(BTN_PIN);
  
  if (mySerial.available()) 
  {
    digitalWrite(LED_PIN, mySerial.read());
  }
  
  if (btn && !prev_btn) 
  {
    led = !led;
    mySerial.write(led);
  }
  prev_btn =btn;
}