#include <Wire.h>

#define SLAVE_ADDR 5           // i2c slave address (1)
#define LED_PIN 4

void setup() {
  // put your setup code here, to run once:
  pinMode(LED_PIN,OUTPUT);
  Wire.begin(SLAVE_ADDR);      // init I2C Slave mode
  Wire.onReceive(receiveEvent); // register event
}

void loop() {
  delay(100);
}
volatile byte msg = 0; 
void receiveEvent(int howMany){
  while (0 < Wire.available()) {
    msg = Wire.read();
    if (msg == 0){
      digitalWrite(LED_PIN, HIGH);
    }else if (msg == 1){
      digitalWrite(LED_PIN, LOW);
    }else{
      msg = 0;
    }
  }
}