Mqtt Classroom

mqtt-classroom

Every XIAO ESP32C6 in the class publishes to one shared Mosquitto broker. This skill wraps the mosquitto_pub / mosquitto_sub commands behind skill.sh.

Broker

You must be on the same WiFi as the broker machine. The address is a private LAN address; it is not reachable from outside the classroom network.

Override the defaults with environment variables if the instructor's IP changes:

export MQTT_HOST=192.168.0.49
export MQTT_PORT=1883

Your device name

Save your board's name once and every command defaults to it:

./skill.sh name seongho    # save
./skill.sh name            # show what is saved
./skill.sh led on          # no id needed any more

It is stored in ~/.mqtt-classroom. MQTT_DEVICE in the environment overrides the saved value for one shell.

The name must match DEVICE_NAME in the board's arduino_secrets.h — that is what decides the board's topic prefix. Two boards sharing a name will fight over the same topics and repeatedly kick each other off the broker, so pick something unique in the class.

Topics

Each board owns a subtree keyed by its name (DEVICE_NAME, or c6- + the last 3 bytes of its MAC if that is left empty). The board prints its name on the serial monitor at boot.

status and led/state are retained, so a fresh subscriber learns the current state immediately instead of waiting for the next change.

Usage

./skill.sh name seongho     # save your board's name (once)
./skill.sh check            # is the broker reachable at all?
./skill.sh devices          # which boards are online
./skill.sh led on           # your board's LED on
./skill.sh led off
./skill.sh led seongho on   # or name another board explicitly
./skill.sh sensor           # stream your board's A0 readings
./skill.sh watch            # every message from your board

Prerequisites

skill.sh needs the mosquitto command line clients:

• Windows — installer from https://mosquitto.org/download/, then add C:\Program Files\mosquitto to PATH. Run skill.sh from Git Bash.
• macOS — brew install mosquitto
• Linux — sudo apt install mosquitto-clients

Dashboard

web/dashboard.html in this repo shows every node, how it is attached to the broker, its live A0 value, and LED buttons. Open the file directly in a browser — no server needed. Point it at another broker with dashboard.html?host=192.168.0.50.

Browser clients

For a web page, connect over WebSockets rather than TCP:

// mqtt.js
const client = mqtt.connect('ws://192.168.0.49:9001')
client.subscribe('classroom/+/sensor/a0')
client.publish('classroom/c6-85ef58/led/set', 'on')

A browser cannot open a raw MQTT TCP socket, so port 1883 will not work from a web page — that is what the 9001 listener exists for.

Troubleshooting