alomqtt.c

#include <WiFi.h>
#include <PubSubClient.h>

const char* ssid = "MEUWIFI";
const char* password = "SENHA";
const char* mqtt_server = "IP_MQTT_SERVER";

WiFiClient espClient;
PubSubClient client(espClient);
String payloadString;

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Topico ");
  Serial.print(topic);
  Serial.print(": ");
  for (int i = 0; i < length; i++) {
    payloadString += (char) payload[i];
  }
  Serial.println(payloadString);
}

void setup() {
  Serial.begin(9600);
  Serial.print("Conectando ao WiFi... ");
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("WiFi conectado!");
  Serial.println("Endereço IP: ");
  Serial.println(WiFi.localIP());
  client.setServer(mqtt_server, 1883);
  client.setCallback(callback);
  client.connect("1");
  while (!client.connected()) {
      Serial.print("Falha:");
      Serial.print(client.state());
      Serial.println(" nova tentativa em 5 segundos");
      delay(5000);
      client.connect("1");
    }
}

void loop() {
    client.publish("iot", "Alo?! Aqui eh o NodeMCU...");
    delay(5000);
}