Subscribing to an MQTT topic

Last updated on 2022-08-30 | Edit this page

Overview

Questions

  • What does one need to get the information published by sensors on the IoT?
  • What is meant by “topic” in terms of MQTT?
  • How can I save the published data to file?

Objectives

  • Connecting to an MQTT broker
  • Subscribing to topics
  • Export data to a file

Our current status

In the previous episodes we have:

  1. Added the URL in the IDE’s preferences to detect the configurations for the ESP32 board
  2. Installed the library required for the DHT sensors
  3. Connected a DHT11 or DHT22 sensor to an ESP32 microcontroller board
  4. Entered the code in a sketch to get temperature readings from the DHT sesions
  5. Connected a Light Dependent Resistor to the same ESP32 microcontroller board
  6. Entered the code in a sketch to get light level readings from the LDR
  7. Entered the code in the sketch to connect to our local network and publish the sensor readings to an MQTT broker

Subscribing to an MQTT topic

At this point your ESP microcontroller board should have a temperature and a light level sensor on it. It should have connected to your local network and to an MQTT broker and it is publishing the readings to a topic to the MQTT broker.

We now need client software to connect to the MQTT broker and subscribe to the topic to which your sensor readings are published. There are quite a few clients available as well as libraries for several programming languages including Python. For this lesson we are going to use MQTTX, a cross platform, open source application. Hopefully you were able to install MQTTX as descripted on the setup page.

Let’s start by running MQTTX. If it is the first time you are running it, a window similar to the image below should open:

Running MQTTX
Running MQTTX

Click on the green + New Connection button and then following window should appear:

New connection in MQTTX
New connection in MQTTX

You now need the information of the MQTT broker that you want to subscribe to. Your instructor should provide you with the correct information. If you are using a local MQTT broker and Raspberry Pi access point your information will be something along the following lines:

  1. Name: raspi-webgui (this can be any name of your choosing)
  2. Client ID: mqttx_46a41312 (this will be different for everyone. You can leave it as is, or change it if you want.)
  3. Host mqtt:// raspap.local
  4. Port: 1883

Click on Connect in the top right hand corner of the window. A small box should appear for a short while, saying CONNECTED, after which the screen should look as follows:

MQTTX connected to broker
MQTTX connected to broker

We are now connected to the MQTT broker and we can now subscribe to a topic.

Click on th +New Subscription button. Enter /JohnSmith/dht22/temperature (replacing JohnSmith with your name as you entered it into the sketch) in the *Topic field and click the Confirm button.

New Subscription
New Subscription

You should now see messages pop up showing the top and a value.

Subscribed
Subscribed

Challenge

Can you add a new subscription to subscribe to the light level that is being published from your ESP32?

Exporting data

MQTTX allows us to export the data that it receives to various formats. Allow MQTTX to run for a while to accumulate some data and then export the data to a JSON file. It is possible to export the data to a CSV but the format appears to be in a somewhat obscure format. It is possible to use convert the JSON to CSV using a converter such as the online converter that is available at www.convertcsv.com/json-to-csv.htm

Key Points

  • Use MQTTX to connect to a broker
  • Subscribed to an MQTT topic
  • Exported data to a file in JSON format