Perhaps you heard about Internet of Things (IoT). The “Things” can be whatever: vehicles, machines, even fridges, assuming they are connected to the internet. But what is on another end of this connection? In some cases – Digital Twins of the “things”. They are kind of virtual twins of real devices. If some characteristics of a device are changed – they are immediately changed in the twin. And vice versa.

Today we will create a digital twin of a device, measuring temperature and humidity, which I described in my previous article How to make a thermometer. We’ll connect it to the Azure IoT Central and link it with its twin.

We will implement the following architecture:

Azure IoT Central Application

First what we need to do is to create an application in Azure IoT Central. Open Azure IoT Central site, select “My apps” tab, and click on the “New application” button. Fill in required fields, following the prompts, appearing on the screen, and create an application.

Device Template

When the application is created, select “Device templates” tab in “App settings” of the application, and create a new template. Select “IoT device” type of the template. You will need to give a name of the template:

Now you need to create a model of your twin. If you are creating a new model from scratch, you will choose a Custom model:

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Then, select your model in the list of models, and press the “Add capability” button. We will add two capabilities:

• Temperature with measurement unit “Degree Celsius”
• Humidity with measurement unit “Percent”.

Add OptimalTemperature property with measurement unit “Degree Celsius”, and make it “Writable” because we need to be able to change this property in a twin.

Let’s add a command just to demonstrate how to send a command (with some parameters) from a twin to a device. Add Reset command with “delay” parameter in its request:

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Do not forget to save your changes.

Now we can create a dashboard, visualizing twin’s properties and telemetry. Select “Views” tab and choose “Visualizing the device” as a new view. Give a name of the view, and add tiles for Temperature and Humidity telemetry, and a tile for the Optimal Temperature property.

You can add (optionally) a view called “Editing device and cloud data”. Try to add a section for the Optimal Temperature property.

Device

Select “Devices” tab on the left side menu and press the “New” button. Enter a device name, a unique device id, and choose a device template you created on the previous step:

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Now select “Administration” tab on the left side menu, and select “Device connection” tab. You will see a page with device connection parameters:

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Save the ID scope in the same text file.

Find a “SAS-IoT_Devices” link on the same page and click on it.

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Copy and save a “Primary key” in your text file.

You have three pieces of the puzzle. Now you need to generate a Device Key. To do that, you shall login to the Azure Portal, and click on the “Cloud Shell” button in the upper navigation menu.

Check that you have azure-iot extension installed, and give the command:

az extension add --name azure-iot

Next, generate a Device Key, using Device ID and Primary Key you saved before:

az iot central device compute-device-key --device-id <device_id> --pk <primary_key>

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Write down the generated Device Key (without quotes).

IoT device

Now we are ready to assemble our IoT device. We’ll be using a Raspberry Pi 4, a DHT22 temperature sensor, and a LCD display. Read my article How to make a thermometer, and wire up the sensor and the LCD display to the Pi as described there.

Turn on power of the Raspberry Pi and log in to it using your favorite client. You will need to install some libraries:

pip3 install adafruit-circuitpython-dht
sudo apt-get install libgpiod2
sudo apt-get install i2c-tools
sudo apt-get install python-smbus
pip3 install azure-iot-device
pip3 install asyncio

The code

Clone the azure-iot-blog GitHub repository into a folder in your Raspberry Pi:

git clone https://github.com/jevgenij-p/azure-iot-blog.git

Change current working directory to iot-central/digital-twin. There are three files:

I2C_LCD_driver.py
send_telemetry.py
set_env.sh

First, we will need to check the I2C address of your LCD using the i2cdetect command, as described in the article How to make a thermometer. If needed, update the LDC address in the I2C_LCD_driver.py file.

Second, open the set_env.sh in your favorite editor. It looks like this:

export PROVISIONING_HOST=global.azure-devices-provisioning.net
export ID_SCOPE=00000000000
export DEVICE_ID=0000000000
export DEVICE_KEY=00000000000000000000000000000000000000000000

Replace zeroes in this file with the values of the ID scope, Device ID and Device Key you saved before. Save the file and run the command to export variables:

source set_env.sh

And, finally, you can run the program, collecting telemetry, and sending it to the digital twin:

python3 send_telemetry.py

Program output will look like this, if everything is done right:

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If you open your Azure IoT Central application, you can see the telemetry, reflected in the digital twin of your device:

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Select “Desired properties” and change the Optimal Temperature value (don’t forget to press Enter in the input field):

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If your program is still running, the LCD display will show you updated “Desired” value:

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Select “Command”, specify some delay in the input field and press the Run button:

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A “Reset” command with delay = 10.5 as an input parameter will be sent from the twin to your IoT device and displayed on the LCD:

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Summary

There are many thing we can do with the digital twin: we can browse raw data, export it to a storage for future analysis, create rules, triggering some actions based on the data, and much more…

I do not want to explain each line in the code – that would take too much time and make the article even more boring 🙂 But I can suggest you to take a look at the Microsoft tutorial, explaining similar code: Tutorial: Create and connect a client application to your Azure IoT Central application.