Example I2C code on Arduino

gistfile1.cpp

// This is the I2C Master

#include <Wire.h>

#define LED_PIN 13
byte x = 0;

void setup()
{
  Wire.begin(); // Start I2C Bus as Master
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);

}
void loop()
{
  
  Wire.beginTransmission(9); // transmit to device #9
  Wire.send(x);              // sends x 
  Wire.endTransmission();    // stop transmitting
  x++;
  if (x > 5) x=0;
  delay(450);
}

gistfile2.cpp

// This is the I2C Slave

#include <Wire.h>

#define LED_PIN 13
#define LED_1 12
#define LED_2 11

int x;

void setup() {
  Wire.begin(9);                // Start I2C Bus as a Slave (Device Number 9)
  Wire.onReceive(receiveEvent); // register event
  
  pinMode(LED_PIN, OUTPUT);
  pinMode(LED_1, OUTPUT);
  pinMode(LED_2, OUTPUT);
  
  digitalWrite(LED_PIN, LOW);
  digitalWrite(LED_1, LOW);
  digitalWrite(LED_2, LOW);
  
  x = 0;
}

void loop() {
  //If value received is 0 blink LED 1
  if (x == 0) {
    digitalWrite(LED_1, HIGH);
    delay(200);
    digitalWrite(LED_1, LOW);
    delay(200);
  }
  //If value received is 1 blink LED 2
  if (x == 1) {
    digitalWrite(LED_2, HIGH);
    delay(200);
    digitalWrite(LED_2, LOW);
    delay(200);
  }
}

void receiveEvent(int howMany) {
  x = Wire.receive();    // receive byte as an integer
}