How to Use MQTT with ESP32: Simple Guide and Example

To use MQTT with ESP32, install the PubSubClient library in Arduino IDE, connect ESP32 to Wi-Fi, then connect to an MQTT broker using the library's client. Publish and subscribe to topics to send and receive messages easily.

Syntax

The basic steps to use MQTT with ESP32 are:

• Include libraries: WiFi.h for Wi-Fi and PubSubClient.h for MQTT.
• Setup Wi-Fi: Connect ESP32 to your Wi-Fi network.
• Setup MQTT client: Define broker address and port, then connect.
• Publish/Subscribe: Use publish() to send messages and subscribe() to listen to topics.

cpp

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

const char* ssid = "yourSSID";
const char* password = "yourPASSWORD";
const char* mqtt_server = "broker.hivemq.com";

WiFiClient espClient;
PubSubClient client(espClient);

void setup_wifi() {
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }
}

void reconnect() {
  while (!client.connected()) {
    if (client.connect("ESP32Client")) {
      client.subscribe("test/topic");
    } else {
      delay(5000);
    }
  }
}

void setup() {
  setup_wifi();
  client.setServer(mqtt_server, 1883);
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
  client.publish("test/topic", "Hello from ESP32");
  delay(2000);
}

Example

This example connects ESP32 to Wi-Fi and an MQTT broker, subscribes to a topic, and publishes a message every 2 seconds.

cpp

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

const char* ssid = "yourSSID";
const char* password = "yourPASSWORD";
const char* mqtt_server = "broker.hivemq.com";

WiFiClient espClient;
PubSubClient client(espClient);

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

void setup_wifi() {
  delay(10);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }
}

void reconnect() {
  while (!client.connected()) {
    if (client.connect("ESP32Client")) {
      client.subscribe("test/topic");
    } else {
      delay(5000);
    }
  }
}

void setup() {
  Serial.begin(115200);
  setup_wifi();
  client.setServer(mqtt_server, 1883);
  client.setCallback(callback);
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
  client.publish("test/topic", "Hello from ESP32");
  delay(2000);
}

Output

Message arrived [test/topic]: Hello from ESP32
Message arrived [test/topic]: Hello from ESP32
...

Common Pitfalls

• Wrong Wi-Fi credentials: ESP32 won't connect if SSID or password is incorrect.
• MQTT broker unreachable: Check broker address and port.
• Not calling client.loop() regularly: This stops MQTT communication.
• Using delay() excessively: Can block MQTT client; use short delays or timers.

cpp

// Wrong way:
void loop() {
  if (!client.connected()) {
    reconnect();
  }
  // Missing client.loop() call
  delay(2000);
}

// Right way:
void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
  delay(2000);
}

Quick Reference

Remember these key points when using MQTT with ESP32:

• Use PubSubClient library for MQTT functions.
• Always connect ESP32 to Wi-Fi before MQTT.
• Call client.loop() frequently in loop().
• Use client.publish(topic, message) to send messages.
• Use client.subscribe(topic) to receive messages.

Key Takeaways

Install and use the PubSubClient library to handle MQTT on ESP32.

Connect ESP32 to Wi-Fi before connecting to the MQTT broker.

Always call client.loop() regularly to maintain MQTT connection.

Use publish() to send and subscribe() to receive MQTT messages.

Check Wi-Fi and broker details carefully to avoid connection issues.