// MASTER BOARD

#include <Wire.h>

unsigned int addressReceiver = 3;

int ledPin = 8;

byte trxData;

void setup() {
  Wire.begin(); //On main board
//  Wire.begin(3); // On the 3rd board
  pinMode(ledPin, OUTPUT);

  trxData = 0;
}

void loop() {
  
  if (trxData == 0)
    trxData = 1;
  else if (trxData == 1)
    trxData = 0;
  digitalWrite(ledPin, HIGH);
  Wire.beginTransmission(addressReceiver);
  Wire.write(trxData);
  Wire.endTransmission();
  delay(100);
  digitalWrite(ledPin, LOW);
  delay(2000);
}