Slave with I2C

#include <Wire.h>
const byte addressSlave = 7;

int rxValue;

void setup()
{
  Wire.begin(addressSlave);
  Wire.onReceive(receiveEvent);
  Serial.begin(115200);
}

void receiveEvent()
{
  rxValue = Wire.read();
  Serial.print("RX: "); Serial.println(rxValue);
}

void loop()
{
  // Later millis() timed WLAN and MQTT code for publishing to AIO
}