MQTT.ino

#include <UIPEthernet.h>
#include <PubSubClient.h>

// Update these with values suitable for your network.
byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
byte server[] = { 104,41,0,81};


void callback(char* topic, byte* payload, unsigned int length) {
  // Aqui trataremos o retorno que vier do servidor.
}

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

void setup()
{
  Ethernet.begin(mac);
  if (client.connect("arduinoClient")) {
    client.publish("NOME_DO_TOPICO","UMA MENSAGEM....");
    client.subscribe("NOME_DO_TOPICO");
  }
}

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