ESP32 connects to the free public MQTT broker

MQTT is a lightweight and flexible IoT message exchange and data transmission protocol, which is dedicated to achieving the balance between flexibility and hardware/network resources for IoT developers.

ESP32 is an upgraded version of ESP8266. In addition to the Wi-Fi module, this module also includes a Bluetooth 4.0 module. The dual-core CPU operates at a frequency of 80 to 240 MHz. It contains two Wi-Fi and Bluetooth modules and various input and output pins. ESP32 is an ideal choice for IoT projects.

In this project, we will connect ESP32 to the free public MQTT server operated and maintained by EMQ X MQTT Cloud, and use the Arduino IDE to program the ESP32. EMQ X Cloud is a secure MQTT IoT cloud service platform launched by EMQ. It provides MQTT 5.0 access service with one-stop operation and maintenance management and a unique isolation environment.

Required IoT components

Arduino configuration

Install ESP32 development board

Click Tools -> Development Board -> Development Board Management -> Search ESP32 -> Install

Install ESP32 development board

Install PubSub client

Project -> Load library -> Library manager... -> Search PubSubClient -> Install PubSubClient by Nick O’Leary

Install PubSub client

ESP32 Pub/Sub diagram

ESP32 Pub/Sub diagram

Programming ESP32 Board with Arduino IDE

Connect to MQTT step by step

  1. First, we will import the WiFi and PubSubClient libraries. The ESP8266WiFi library can connect ESP32 to Wi-Fi networks, and the PubSubClient library can connect ESP32 to the MQTT server to publish messages and subscribe to topics.

    #include <WiFi.h>
    #include <PubSubClient.h>
    
  2. Set the Wi-Fi name and password, as well as the MQTT server connection address and port, and set the topic to "esp32/test".

    // WiFi
    const char *ssid = "mousse"; // Enter your WiFi name
    const char *password = "qweqweqwe";  // Enter WiFi password
    
    // MQTT Broker
    const char *mqtt_broker = "broker.emqx.io";
    const char *topic = "esp32/test";
    const char *mqtt_username = "emqx";
    const char *mqtt_password = "public";
    const int mqtt_port = 1883;
    
  3. Open a serial connection to output the results of the program and connect to the Wi-Fi network.

    // Set software serial baud to 115200;
    Serial.begin(115200);
    // connecting to a WiFi network
    WiFi.begin(ssid, password);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.println("Connecting to WiFi..");
    }
    
  4. Use PubSubClient to connect to the public MQTT Broker.

    client.setServer(mqtt_broker, mqtt_port);
    client.setCallback(callback);
    while (!client.connected()) {
        String client_id = "esp32-client-";
        client_id += String(WiFi.macAddress());
        Serial.printf("The client %s connects to the public mqtt broker\n", client_id.c_str());
        if (client.connect(client_id.c_str(), mqtt_username, mqtt_password)) {
            Serial.println("Public emqx mqtt broker connected");
        } else {
            Serial.print("failed with state ");
            Serial.print(client.state());
            delay(2000);
        }
    }
    
  5. After the MQTT server is successfully connected, ESP32 will publish messages to the MQTT server of esp/test and subscribe to the topic messages of esp/test.

    // publish and subscribe
    client.publish(topic, "Hi EMQ X I'm ESP32 ^^");
    client.subscribe(topic);
    
  6. Set the callback function to print the topic name to the serial port and print the message received from the esp32/test topic.

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

Full code

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

// WiFi
const char *ssid = "mousse"; // Enter your WiFi name
const char *password = "qweqweqwe";  // Enter WiFi password

// MQTT Broker
const char *mqtt_broker = "broker.emqx.io";
const char *topic = "esp32/test";
const char *mqtt_username = "emqx";
const char *mqtt_password = "public";
const int mqtt_port = 1883;

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
 // Set software serial baud to 115200;
 Serial.begin(115200);
 // connecting to a WiFi network
 WiFi.begin(ssid, password);
 while (WiFi.status() != WL_CONNECTED) {
     delay(500);
     Serial.println("Connecting to WiFi..");
 }
 Serial.println("Connected to the WiFi network");
 //connecting to a mqtt broker
 client.setServer(mqtt_broker, mqtt_port);
 client.setCallback(callback);
 while (!client.connected()) {
     String client_id = "esp32-client-";
     client_id += String(WiFi.macAddress());
     Serial.printf("The client %s connects to the public mqtt broker\n", client_id.c_str());
     if (client.connect(client_id.c_str(), mqtt_username, mqtt_password)) {
         Serial.println("Public emqx mqtt broker connected");
     } else {
         Serial.print("failed with state ");
         Serial.print(client.state());
         delay(2000);
     }
 }
 // publish and subscribe
 client.publish(topic, "Hi EMQ X I'm ESP32 ^^");
 client.subscribe(topic);
}

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

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

Running and testing

  1. Use Arduino to upload the complete code and power on the esp32.

  2. Open the serial monitor, select 115200 baud rate, and check the ESP32 connection status.

    check the ESP32 connection status

  3. Use the MQTT X client to connect to the public MQTT server and publish messages to ESP32.

    MQTT X client

Summary

So far, we have successfully connected ESP32 to the public MQTT server provided by EMQ X Cloud. In this project, we simply connect ESP32 to the MQTT server. This is just one of ESP32's basic capabilities. ESP32 can actually connect to various IoT sensors and report sensor data to the MQTT server.

Next, we will publish more articles about IoT development and ESP32, so stay tuned.

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