#include <Wire.h>

void setup() {
  pinMode(5, OUTPUT);     // Setup the built-in LED
  Wire.begin(0x04);       // Set this device's I2C address to 0x04
  Wire.onReceive(receiveEvent);
  Serial.begin(115200);
}

void loop() {
  delay(100);  // Do nothing, just wait for I2C commands
}

void receiveEvent(int howMany) {
  while (Wire.available()) {
    int c = Wire.read();  // Read byte
    if (c == 1) {
      digitalWrite(5, HIGH);  // Turn LED on
      delay(2000);            // Keep it on for 1 second
      digitalWrite(5, LOW);   // Then turn it off
    }
  }
}