Backend

There are many options for a backend, but a nice solution I recommend is ThingsBoard.

There is a demo version at https://demo.thingsboard.io/ which is perfect for experimenting.

You can easily deliver data from your thing via MQTT as explained here

mosquitto_pub -d -q 1 -h "$THINGSBOARD_HOST_NAME" -p "1883" -t "v1/devices/me/telemetry" -u "$ACCESS_TOKEN" -m {"temperature":25}

A convenient multiplatform tool for experimenting is MQTTX.

The configuration is the one below where username is the access token

Note that in some cases MQTT protocol is filtered out, so be sure you are in a network that can easily deal with MQTT.

Once you are satisfied by your experiments, you can easily move to a real thing with the follwoing code.

You need to install the following libraries in your Arduino IDE:

• WiFi (Standard library for ESP32)
• PubSubClient by Nick O'Leary

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

// --- Network Configuration ---
const char* WIFI_SSID = "";             // <-- REPLACE with your Wi-Fi network name
const char* WIFI_PASS = "";         // <-- REPLACE with your Wi-Fi password

// --- ThingsBoard & MQTT Configuration ---
// ThingsBoard Cloud MQTT Server Address (use your instance if self-hosted)
const char* TB_SERVER = "demo.thingsboard.io";        // Or your TB instance (e.g., 'thingsboard.cloud')
const int TB_PORT = 1883;                             // Default MQTT port

// ThingsBoard Authentication: The ACCESS TOKEN is used as the MQTT USERNAME
const char* TB_ACCESS_TOKEN = ""; // <-- REPLACE with your actual Device Access Token

// Topics for publishing telemetry data and attributes
const char* TELEMETRY_TOPIC = "v1/devices/me/telemetry"; // The standard topic for device data
const char* ATTRIBUTES_TOPIC = "v1/devices/me/attributes"; // Topic for device attributes (e.g., firmware version)

// --- Initialize Clients ---
WiFiClient espClient; 
PubSubClient client(espClient);

// Timing variables for publishing data
long lastMsg = 0;
const int PUBLISH_INTERVAL = 10000; // Publish every 10 seconds (10000 ms)

// Function prototypes
void setup_wifi();
void reconnect_mqtt();

// =========================================================================
// SETUP
// =========================================================================
void setup() {
  Serial.begin(115200);
  setup_wifi();

  // Set the MQTT server
  client.setServer(TB_SERVER, TB_PORT);
  // Note: We don't need a callback here since we are only publishing (for now).
}

// =========================================================================
// LOOP
// =========================================================================
void loop() {
  // Check and maintain the MQTT connection
  if (!client.connected()) {
    reconnect_mqtt();
  }

  // PubSubClient requires loop() to be called frequently
  client.loop();

  long now = millis();
  if (now - lastMsg > PUBLISH_INTERVAL) {
    lastMsg = now;
    publishTelemetry();
  }
}

// =========================================================================
// HELPER FUNCTIONS
// =========================================================================

// Connect to Wi-Fi
void setup_wifi() {
  delay(10);
  Serial.print("Connecting to ");
  Serial.println(WIFI_SSID);

  WiFi.begin(WIFI_SSID, WIFI_PASS);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  Serial.println("\nWiFi connected");
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());
}

// Reconnect to ThingsBoard MQTT Broker
void reconnect_mqtt() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection to ThingsBoard...");

    // --- CRITICAL AUTHENTICATION STEP ---
    // ThingsBoard uses the ACCESS TOKEN as the MQTT USERNAME.
    // The password field is typically left empty (NULL).
    if (client.connect(TB_ACCESS_TOKEN, TB_ACCESS_TOKEN, NULL)) {
      Serial.println("connected!");
      // Optionally, send an attribute (e.g., firmware version) upon connection
      client.publish(ATTRIBUTES_TOPIC, "{\"firmware_version\":\"1.0.0\"}");

    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" trying again in 5 seconds...");
      delay(5000);
    }
  }
}

// Function to generate and publish telemetry data
void publishTelemetry() {
  // 1. Generate a simulated sensor reading (replace with real sensor data)
  // Generates a random float between 20.0 and 30.0
  float temperature = random(200, 300) / 10.0; 
  float humidity = random(400, 600) / 10.0;

  // 2. Format the data as a JSON string (mandatory for ThingsBoard telemetry)
  // {"temperature": 25.5, "humidity": 45.0}
  String payload = "{\"temperature\":" + String(temperature) + 
                   ",\"humidity\":" + String(humidity) + "}";

  // 3. Publish the JSON payload
  // The topic MUST be "v1/devices/me/telemetry"
  if (client.publish(TELEMETRY_TOPIC, payload.c_str())) {
    Serial.print("Published telemetry: ");
    Serial.println(payload);
  } else {
    Serial.println("Failed to publish telemetry.");
  }
}