MQTT_Ethernet.ino

/*
 Publishing in the callback

  - connects to an MQTT server
  - subscribes to the topic "inTopic"
  - when a message is received, republishes it to "outTopic"

  This example shows how to control pins using Ethernet shield via MQTT

*/

#include <SPI.h>
#include <Ethernet.h>
#include <PubSubClient.h>

// Update these with values suitable for your network.
byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
IPAddress ip(192, 168, 1, 100);
IPAddress server(192, 168, 1, 117);

const int lightPin = 8;

// Callback function header
void callback(char* topic, byte* payload, unsigned int length);

EthernetClient ethClient;
PubSubClient client(server, 1883, callback, ethClient);

// Callback function
void callback(char* topic, byte* payload, unsigned int length) {
    
  //turn the LED ON if the payload is '1' and publish to the MQTT server a confirmation message
  if(payload[0] == '1'){
    digitalWrite(lightPin, HIGH);
    client.publish("outTopic", "Light On"); }
    
  //turn the LED OFF if the payload is '0' and publish to the MQTT server a confirmation message
  if (payload[0] == '0'){
    digitalWrite(lightPin, LOW);
    client.publish("outTopic", "Light Off");
    }
} // void callback


void setup()
{
  pinMode(lightPin, OUTPUT);
  digitalWrite(lightPin, LOW);
  
  Ethernet.begin(mac, ip);
  if (client.connect("arduinoClient")) {
    client.publish("outTopic","hello world");
    client.subscribe("inTopic");
  }
}

void loop()
{
  client.loop();
}