Arduino_Slave_IO_Expander.ino

// Arduino as IO - Extender (Slave_Code)
// by Varad Kulkarni <http://www.microcontrollershub.com>
// Created on 16 March 2018

#include <Wire.h>

void setup() 
{
  Wire.begin(0x40);                 // Initialize Arduino as slave with addres 0x40
  Wire.onReceive(receiveEvent);     // Register event
  Serial.begin(9600);               // Start serial for debug
  pinMode(8,OUTPUT);                // Use LED_1 connected tompin 8 as indicator
  digitalWrite(8,LOW);              // Initialize LED_1 in OFF state
  pinMode(9,OUTPUT);                // Use LED_2 connected tompin 9 as indicator
  digitalWrite(9,LOW);              // Initialize LED_2 in OFF state
}

void loop() 
{
}

// function that executes whenever data is received from master

void receiveEvent(int howMany) 
{
  while (0 < Wire.available())              // loop through all but the last
  {  
    Serial.println("data received"); 
    char cReceivedData = Wire.read();       // receive byte as an char
    Serial.println(cReceivedData);          // print the character
    if('1' == cReceivedData)                // If '1' is received, turn LED_1 OFF
    {
      digitalWrite(8,HIGH);
    }
    else if('2' == cReceivedData)           // If '2' is received, turn LED_1 ON
    {
      digitalWrite(8,LOW);
    }
    else if('3' == cReceivedData)           // If '3' is received, turn LED_2 ON
    {
      digitalWrite(9,HIGH);
    }
    else if('4' == cReceivedData)           // If '4' is received, turn LED_2 ON
    {
      digitalWrite(9,LOW);
    }
  }

}