Subscribe to commands and report the light state

Use a separate ESP32 subscriber to turn a simulated LED on and off in response to the companion motion publisher.

Broker Bex: I want you to compare the received command with the LED and the state it reports., your practice guide

Broker Bex: I want you to compare the received command with the LED and the state it reports.
Predict the reading, then compare it with the measurement.

Wokwi ESP32

Third party Tool

Use a separate ESP32 subscriber to turn a simulated LED on and off in response to the companion motion publisher.

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

883 bytes · Circuit connections

Download

sketch.ino

1,992 bytes · ESP32 program

Download

// Use the same unique ROOM_ID (up to 24 characters) in both companion labs.
#include <WiFi.h>
#include <PubSubClient.h>

const char* ssid = "Wokwi-GUEST";
const char* password = "";
const char* mqtt_server = "test.mosquitto.org";

WiFiClient espClient;
PubSubClient client(espClient);

#define LED_PIN 2
#define ROOM_ID "iotclass-w3-20260909"

char light_command_topic[96];
char light_state_topic[96];

void mqtt_callback(char* topic, byte* payload, unsigned int length) {
  String message = "";
  for (int i = 0; i < length; i++) {
    message += (char)payload[i];
  }

Serial.print("Received command: ");
  Serial.println(message);

if (message == "ON") {
    digitalWrite(LED_PIN, HIGH);
    client.publish(light_state_topic, "ON", true);
    Serial.println("Light turned ON");
  } else if (message == "OFF") {
    digitalWrite(LED_PIN, LOW);
    client.publish(light_state_topic, "OFF", true);
    Serial.println("Light turned OFF");
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);

sprintf(light_command_topic, "home/%s/light/command", ROOM_ID);
  sprintf(light_state_topic, "home/%s/light/state", ROOM_ID);

WiFi.begin(ssid, password, 6);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("\nWi-Fi connected");

client.setServer(mqtt_server, 1883);
  client.setCallback(mqtt_callback);

reconnect();
}

void reconnect() {
  while (!client.connected()) {
    Serial.print("Connecting to MQTT...");
    if (client.connect("iotclass-light-" ROOM_ID)) {
      Serial.println(" Connected");

// Subscribe to light commands
      client.subscribe(light_command_topic);
      Serial.print("Subscribed to: ");
      Serial.println(light_command_topic);

// Publish initial state
      client.publish(light_state_topic, "OFF", true);
    } else {
      delay(5000);
    }
  }
}

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

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
Open Wokwi, paste this lab’s diagram.json and sketch.ino, and add PubSubClient in Library Manager. Inspect the GPIO 2 LED path.
You will see
GPIO 2 drives the LED through a 220 ohm resistor, with its cathode connected to ground.
Why it matters
The subscriber turns received commands into a visible output, separate from the sensor.
Step 2
Do
Choose the same unique ROOM_ID in this sketch and /labs/home-motion-publish.html; inspect the callback and topic strings before starting.
You will see
The callback accepts ON and OFF; the subscribed path ends in light/command and the reported path in light/state.
Why it matters
Commands and reported state have different meanings and therefore use separate topics.
Step 3
Do
Start this subscriber first and open Serial Monitor. Then start the companion motion publisher in a second tab with the matching ROOM_ID.
You will see
Wi-Fi connected and Subscribed to: home/ROOM_ID/light/command appear; the LED starts off.
Why it matters
A non-retained command reaches a currently connected subscriber; starting in order avoids missing the first command.
Step 4
Do
In the publisher tab, click the PIR and Simulate motion. Return to this subscriber’s Serial Monitor.
You will see
Received command: ON and Light turned ON appear.
Why it matters
The callback’s output is evidence of delivery and actuation, beyond a successful broker connection.
Step 5
Do
Inspect the subscriber’s LED while the motion event is active.
You will see
The green LED is illuminated.
Why it matters
Checking the output alongside the log distinguishes the message from the visible simulated action.
Step 6
Do
Wait for the publisher’s motion to clear and its 30-second timer to expire. Read this subscriber’s Serial Monitor.
You will see
Received command: OFF and Light turned OFF appear; the LED goes dark.
Why it matters
The subscriber acts on a delivered OFF command. Its retained state report is a snapshot, not a history of the motion event.

Chapter checks

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

What is the main design value of MQTT publish-subscribe in an IoT deployment?
Return to the chapter’s knowledge check
A dashboard needs all temperature readings from greenhouse 2, but not other sites or other signal types. Which subscription is the best fit?
Return to the chapter’s knowledge check

Return to MQTT Publish-Subscribe Basics · 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

Chapter checks

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6