How to use Developer Tools → MQTT to test an MQTT connection

I’m trying to get started with ESP01s modules and connecting them via MQTT to on a RPi 3b+.

The ESP module is showing as connected when I go into Supervisor → Mosquitto Broker → Log.

The sketch I’ve currently loaded into the ESP module was from the first Arduino library I found via google for MQTT it’s called mqtt_esp8266

The problem I’m facing is really not knowing how MQTT really works and a lack of entry level documentation on it. Considering I’m new to, MQTT and am working with examples that I’m not sure don’t have mistakes it’s been pretty damn frustrating trying to get started. I reckon I’ve sank about 15 or 20 hours into just trying to get the onboard LED to be controlled through Home Assistant, I thought the safest place to start was through the Developer Tools.

If anyone could help me with constructing the “inTopic” packet I should send to the ESP module to get the LED to blink I’d be very grateful. It seems to me that the developers of this stuff don’t really do documentation and it’s left up to online communities for people to get how to’s for simple stuff. Pasted the sketch below in case there’s something wrong in it.

 Basic ESP8266 MQTT example
 This sketch demonstrates the capabilities of the pubsub library in combination
 with the ESP8266 board/library.
 It connects to an MQTT server then:
  - publishes "hello world" to the topic "outTopic" every two seconds
  - subscribes to the topic "inTopic", printing out any messages
    it receives. NB - it assumes the received payloads are strings not binary
  - If the first character of the topic "inTopic" is an 1, switch ON the ESP Led,
    else switch it off
 It will reconnect to the server if the connection is lost using a blocking
 reconnect function. See the 'mqtt_reconnect_nonblocking' example for how to
 achieve the same result without blocking the main loop.
 To install the ESP8266 board, (using Arduino 1.6.4+):
  - Add the following 3rd party board manager under "File -> Preferences -> Additional Boards Manager URLs":
  - Open the "Tools -> Board -> Board Manager" and click install for the ESP8266"
  - Select your ESP8266 in "Tools -> Board"

#include <ESP8266WiFi.h>
#include <PubSubClient.h>

// Update these with values suitable for your network.

const char* ssid = "........";
const char* password = "........";
const char* mqtt_server = "";

WiFiClient espClient;
PubSubClient client(espClient);
unsigned long lastMsg = 0;
#define MSG_BUFFER_SIZE	(50)
char msg[MSG_BUFFER_SIZE];
int value = 0;

void setup_wifi() {

  // We start by connecting to a WiFi network
  Serial.print("Connecting to ");

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {


  Serial.println("WiFi connected");
  Serial.println("IP address: ");

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

  // Switch on the LED if an 1 was received as first character
  if ((char)payload[0] == '1') {
    digitalWrite(BUILTIN_LED, LOW);   // Turn the LED on (Note that LOW is the voltage level
    // but actually the LED is on; this is because
    // it is active low on the ESP-01)
  } else {
    digitalWrite(BUILTIN_LED, HIGH);  // Turn the LED off by making the voltage HIGH


void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Create a random client ID
    String clientId = "ESP8266Client-";
    clientId += String(random(0xffff), HEX);
    // Attempt to connect
    if (client.connect(clientId.c_str())) {
      // Once connected, publish an announcement...
      client.publish("outTopic", "hello world");
      // ... and resubscribe
    } else {
      Serial.print("failed, rc=");
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying

void setup() {
  pinMode(BUILTIN_LED, OUTPUT);     // Initialize the BUILTIN_LED pin as an output
  client.setServer(mqtt_server, 1883);

void loop() {

  if (!client.connected()) {

  unsigned long now = millis();
  if (now - lastMsg > 2000) {
    lastMsg = now;
    snprintf (msg, MSG_BUFFER_SIZE, "hello world #%ld", value);
    Serial.print("Publish message: ");
    client.publish("outTopic", msg);

Actually, you should add “inTopic” in the topic and “1” as the payload.
Int he supbscibe you can use “#” to listen to everything to get an idea of what is sent around.

Is the Server “” the correct broker URL and are you shure you need no credentials for it?

With listening to “#” you will get a better idea I guess.

Hey I finally figured out that there were problems with how I’d configured Mosquitto and. I needed to g back and start from scratch. Be careful what tutorials you follow.

what were the problems with your configuration, and what tutorial was incorrect?