Temperature & Humidity Monitoring

One of the earliest projects many people build is temperature & humidity measurement, we've mentioned this in the past as a simple project.

This project is a more complex version of that, but only in terms of software. What we're going to build is a simple device which will:

The end result will be that I can deploy the same code on a bunch of boards, and each will send their temperature & humidity data to a central queue. From there I can record the values and end up with a useful graphing solution in a hands-free fashion.


The hardware is precisely the same as our simple project:

  • 1 x WeMos Mini D1
    • Approximately cost €2.50.
  • 1 x DHT22 Shield
    • Approximate cost €3.00.

The shields available on AliExpress vary somewhat, but mine connects the sensor to pin D4 (which is plainly visible when looking at the traces on the board). Because I'm using a shield the connection is small and neat, and the end result is just powered by a typical USB phone-charger:

WeMos Mini D1 and DHT22 shield


This project requires three pieces of software:

  • The actual software running on the board itself.
    • This reads the local environment and publishes the value on the MQ bus.
  • A simple piece of software to listen to the queue, and record the values to a local database.
  • A viewing-application which will read the values from the database and let you see a pretty graph.

All the code is bundled together in a git repository, you can access all the source by cloning the repo involved, and then looking at the project itself:


If you've previously configured WiFi details the device will connect automatically, and begin showing the date/time, along with the departure times.

If you've never configured the device it will instead notice that, and configure itself as an access-point. This means you can use your mobile phone, or other WiFi-connected device, to connect to this access-point and choose the local access-point it should become a member of.

At that point the device itself is working, but it will be only sending updates once a minute to the message-bus. Without something listening for those updates you've not got anything useful!

To make things useful you'll need to have a local machine which can also connect to the bus. That will just sit idle, waiting for updates to be received on the "temperature" topic. When such a message arrives it needs to be saved to a database so that you can query it for display, or similar, in the future.

With the data recorded to a local database you can now query it. I write a simple CGI script which will present a list of devices and draw interactive graphs of both temperature & humidity.

Recent Changes

One of the main goals of this project was that I could burn the same code into a bunch of different ESP-devices, and they would each submit their data to the message-queue.

The first device was deployed for several weeks before I got round to setting up more of them, at that point I learned that when a client connects to the queue it must announce itself with a unique name.

So now when clients connect to the queue their name is suffixed with the MAC address of the on-board WiFi device.