Arduino_Master_IO_Expander.ino

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

#include <Wire.h>

//------------ Serial commands ------------ 
String LED_1_ON_Command   = "LED 1 ON";        // I2C Command = 1
String LED_1_OFF_Command  = "LED 1 OFF";       // I2C Command = 2
String LED_2_ON_Command   = "LED 2 ON";        // I2C Command = 3
String LED_2_OFF_Command  = "LED 2 OFF";       // I2C Command = 4

void setup() 
{
  Serial.begin(9600);               // Initialize Serial communication at 9600 baud
  Wire.begin();                     // Initialize Arduino as Master
}

void loop() 
{
  if(Serial.available())
  {
    String ReceivedString = Serial.readString();
    Serial.println("****** Serial Data recevied  ******");
    Serial.println(ReceivedString);
    delay(200);

    if(LED_1_ON_Command == ReceivedString)
    {
      Wire.beginTransmission(0x40);    // Transmit to Slave device #0x40
      Wire.write('1');                 // Sends one byte as '1'
      Wire.endTransmission();          // End transmition
      delay(100);
    }
    else if(LED_1_OFF_Command == ReceivedString)
    {
      Wire.beginTransmission(0x40);    // Transmit to Slave device #0x40
      Wire.write('2');                 // Sends one byte as '2'
      Wire.endTransmission();          // End transmition
      delay(100);
    }
    else if(LED_2_ON_Command == ReceivedString)
    {
      Wire.beginTransmission(0x40);    // Transmit to Slave device #0x40
      Wire.write('3');                 // Sends one byte as '3'
      Wire.endTransmission();          // End transmition
      delay(100);
    }
    else if(LED_2_OFF_Command == ReceivedString)
    {
      Wire.beginTransmission(0x40);    // Transmit to Slave device #0x40
      Wire.write('4');                 // Sends one byte as '4'
      Wire.endTransmission();          // End transmition
      delay(100);
    }
    
  }
}