Publish temperature on a unique MQTT topic

Publish changing DHT22 readings every five seconds under the ESP32’s randomly generated MQTT topic root.

Broker Bex: I want you to follow one reading from its sensor value to its own topic and distinguish a publish result from subscriber delivery., your practice guide

Broker Bex: I want you to follow one reading from its sensor value to its own topic and distinguish a publish result from subscriber delivery.
Predict the reading, then compare it with the measurement.

Wokwi ESP32

Third party Tool

Publish changing DHT22 readings every five seconds under the ESP32’s randomly generated MQTT topic root.

Tier 1 · Web · No account

Version tested: Compiled and booted by the Wokwi gate (ESP32 Arduino core 3.3.11); captured in the Wokwi web editor on 2026-09-09. Date: 2026-09-09.

Open the ESP32 editor, paste diagram.json, then paste sketch.ino.

Open Wokwi to paste in the files (new tab)

Get the files

Use both prepared files. This is a paste-in setup; saving a project requires a Wokwi account.

diagram.json

1,065 bytes · Circuit connections

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{
  "version": 1,
  "author": "IoTClass",
  "editor": "wokwi",
  "parts": [
    {
      "type": "wokwi-esp32-devkit-v1",
      "id": "esp",
      "top": 100,
      "left": 50,
      "attrs": {}
    },
    {
      "type": "wokwi-dht22",
      "id": "dht",
      "top": -100,
      "left": 0,
      "attrs": {
        "temperature": "24",
        "humidity": "40"
      }
    },
    {
      "type": "wokwi-resistor",
      "id": "pullup",
      "top": -10,
      "left": -90,
      "attrs": {
        "value": "10000"
      }
    }
  ],
  "connections": [
    [
      "esp:TX0",
      "$serialMonitor:RX",
      "",
      []
    ],
    [
      "esp:RX0",
      "$serialMonitor:TX",
      "",
      []
    ],
    [
      "dht:VCC",
      "esp:3V3",
      "red",
      []
    ],
    [
      "dht:GND",
      "esp:GND.1",
      "black",
      []
    ],
    [
      "dht:SDA",
      "esp:D4",
      "green",
      []
    ],
    [
      "esp:3V3",
      "pullup:1",
      "red",
      []
    ],
    [
      "pullup:2",
      "dht:SDA",
      "green",
      []
    ]
  ]
}

sketch.ino

4,028 bytes · ESP32 program

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

// Wi-Fi credentials
const char* ssid = "Wokwi-GUEST";
const char* password = "";

// MQTT Broker settings
const char* mqtt_server = "test.mosquitto.org";
const int mqtt_port = 1883;

// Generated on this ESP32 at boot: 48 random bits, printed in setup().
char learner_suffix[13];
char mqtt_client_id[24];

// DHT22 sensor
#define DHTPIN 4
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);

// MQTT topics
char topic_root[40];
char topic_temp[56];
char topic_humidity[56];
char topic_status[56];
char topic_command[56];

WiFiClient espClient;
PubSubClient client(espClient);

// Forward declaration (defined below loop)
void mqtt_callback(char* topic, byte* payload, unsigned int length);

unsigned long lastPublish = 0;
const long publishInterval = 5000; // 5 seconds

void make_lab_identity() {
  uint32_t random_high = esp_random();
  uint32_t random_low = esp_random();

snprintf(learner_suffix, sizeof(learner_suffix), "%08lX%04lX",
           (unsigned long)random_high,
           (unsigned long)(random_low & 0xFFFF));
  snprintf(mqtt_client_id, sizeof(mqtt_client_id), "iotlab-%s", learner_suffix);
  snprintf(topic_root, sizeof(topic_root), "iotclass/lab1/%s", learner_suffix);
  snprintf(topic_temp, sizeof(topic_temp), "%s/temperature", topic_root);
  snprintf(topic_humidity, sizeof(topic_humidity), "%s/humidity", topic_root);
  snprintf(topic_status, sizeof(topic_status), "%s/status", topic_root);
  snprintf(topic_command, sizeof(topic_command), "%s/command", topic_root);
}

void setup_wifi() {
  Serial.println("\nConnecting to Wi-Fi...");
  WiFi.begin(ssid, password);

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

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

void reconnect_mqtt() {
  while (!client.connected()) {
    Serial.print("Connecting to MQTT broker...");

if (client.connect(mqtt_client_id)) {
      Serial.println(" Connected");

// Publish online status as retained message
      client.publish(topic_status, "online", true);

// Subscribe to commands (optional)
      client.subscribe(topic_command);
    } else {
      Serial.print(" Failed, rc=");
      Serial.println(client.state());
      delay(5000);
    }
  }
}

void setup() {
  Serial.begin(115200);
  dht.begin();

setup_wifi();
  make_lab_identity();

Serial.print("Unique MQTT ClientID: ");
  Serial.println(mqtt_client_id);
  Serial.print("Your Lab 1 topic root: ");
  Serial.println(topic_root);

client.setServer(mqtt_server, mqtt_port);
  client.setCallback(mqtt_callback);
}

void mqtt_callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message received on ");
  Serial.print(topic);
  Serial.print(": ");

String message = "";
  for (int i = 0; i < length; i++) {
    message += (char)payload[i];
  }
  Serial.println(message);
}

void loop() {
  if (!client.connected()) {
    reconnect_mqtt();
  }
  client.loop();

unsigned long now = millis();
  if (now - lastPublish >= publishInterval) {
    lastPublish = now;

// Read sensor
    float humidity = dht.readHumidity();
    float temperature = dht.readTemperature();

if (isnan(humidity) || isnan(temperature)) {
      Serial.println("Failed to read from DHT sensor!");
      return;
    }

// Publish temperature with QoS 0
    char tempStr[8];
    dtostrf(temperature, 6, 2, tempStr);
    bool temp_success = client.publish(topic_temp, tempStr, false);
    Serial.print("Temperature: ");
    Serial.print(tempStr);
    Serial.print("C ");
    Serial.println(temp_success ? "OK" : "FAIL");

// Publish humidity with QoS 0
    char humStr[8];
    dtostrf(humidity, 6, 2, humStr);
    bool hum_success = client.publish(topic_humidity, humStr, false);
    Serial.print("Humidity: ");
    Serial.print(humStr);
    Serial.print("% ");
    Serial.println(hum_success ? "OK" : "FAIL");

Serial.println("---");
  }
}

Use the launch button above to open the ESP32 editor in Wokwi.
Select the editor’s diagram.json tab and replace all its text with the supplied diagram.json.
Select the sketch.ino tab, replace all its text with the supplied sketch.ino, then click Start Simulation.

Steps

Screens captured against Wokwi ESP32 Compiled and booted by the Wokwi gate (ESP32 Arduino core 3.3.11); captured in the Wokwi web editor on 2026-09-09 on 2026-09-09; the tool may have moved on — the text steps are the contract.

Step 1
Do
Paste the diagram and sketch into Wokwi ESP32. Install PubSubClient and DHT sensor library in Library Manager. Inspect the sensor wiring.
You will see
The DHT22 data line uses GPIO 4 with a 10 kΩ pull-up to 3.3 V; its supply and ground are connected.
Why it matters
The host’s publisher role starts with a valid sensor reading. Wiring and library setup are part of making that reading reproducible.
Step 2
Do
Start the simulation at 115200 baud and wait for Wi-Fi and broker connection.
You will see
Wi-Fi connected appears, followed by a unique iotlab- ClientID, an iotclass/lab1/ topic root with a 12-hex-digit suffix, and Connecting to MQTT broker... Connected.
Why it matters
The broker accepts a client identity and routes publications by topic. The source’s per-boot identifiers prevent ordinary classroom ClientID collisions.
Step 3
Do
Wait for the first sensor publication and keep two consecutive output groups.
You will see
At the supplied settings the sketch prints Temperature: 24.00C OK and Humidity: 40.00% OK, separated into groups by ---, roughly every five seconds.
Why it matters
The ESP32 is the publisher; a dashboard would be a subscriber. OK here is PubSubClient’s publish result, not proof that a dashboard received the reading.
Step 4
Do
Select the DHT22 and set temperature to 30 °C while keeping humidity at 40%.
You will see
The next valid output group changes to Temperature: 30.00C OK while humidity remains 40.00%.
Why it matters
Changing one quantity makes the payload’s meaning visible. The host keeps sensing, broker routing, and display responsibilities separate.
Step 5
Do
Set humidity to 60% without changing the 30 °C temperature.
You will see
The next group reports Humidity: 60.00% OK alongside Temperature: 30.00C OK.
Why it matters
Temperature and humidity use separate topic suffixes, allowing subscribers to request the quantity they need.
Step 6
Do
Compare make_lab_identity and the publish calls in the sketch with the topic root printed in Serial Monitor.
You will see
The root has /temperature, /humidity, /status, and /command suffixes. Sensor publishes pass false for retained state, while the online status uses true.
Why it matters
Topic naming and retained state are distinct choices. This starter publishes sensor values at QoS 0; the Boolean argument is the retain flag, not a QoS setting.
Step 7
Do
Restore the DHT22 to 24 °C and 40% RH, confirm the readings, then stop and restart once.
You will see
The original sensor values return. Restart prints a new random ClientID and topic root, so the previous run’s subscription would need updating.
Why it matters
Public broker sessions are temporary teaching infrastructure. The host explains why availability, isolation, and production security cannot be inferred from a successful public test.

Chapter checks

These questions refer to the chapter’s examples. Use the return links to review their answers.

In an MQTT-based temperature monitoring system, an ESP32 sensor node reads temperature data and sends it to a broker. A Python dashboard running on a laptop displays the readings. Which statement correctly identifies each component's role?
Return to the chapter’s knowledge check
This chapter's 'Common Mistake' warning lists four dangers of using test.mosquitto.org in a production deployment. Which of the following is one of them?
Return to the chapter’s knowledge check

Return to MQTT Publisher-Subscriber Setup · Browse Labs

Wokwi ESP32

Get the files

diagram.json

sketch.ino

Steps

1 Step 1

2 Step 2

3 Step 3

4 Step 4

5 Step 5

6 Step 6

7 Step 7

Chapter checks

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7