A microcontroller that supports MicroPython (e.g. ESP8266, ESP32, Raspberry Pi Pico W, etc.)
A DHT22 temperature and humidity sensor
A breadboard and some jumper wires to create the circuit

Circuit

Install the microconller and the DHT22 sensor on different parts of the breadboard. Make the following connections with jumper wires:

from a microcontroller power pin (3.3V or 5V) to the left pin of the DHT22 sensor.
from a microcontroller GND pin to the right pin of the DHT22 sensor.
from any available microcontroller GPIO pin to the middle pin of the DHT22 sensor. If the DHT22 sensor has 4 pins instead of 3, use the one on the left, next to the pin receiving power.

The following diagram shows a possible wiring for this circuit using an ESP8266 microcontroller and the 4-pin variant of the DHT22. In this diagram the data pin of the DHT22 sensor is connected to pin D2 of the ESP8266, which is assigned to GPIO #4. Note that the location of the pins in the microcontroller board will vary depending on which microcontroller you use.

Installation

Edit config.py as follows:

Set the DHT22_PIN variable to the GPIO pin number connected to the sensor's data pin. Make sure you consult the documentation for your microcontroller to learn what number you should use for your chosen GPIO pin. In the example diagram above, the value should be 4.
Enter your Wi-Fi SSID name and password in this file.

Install MicroPython on your microcontroller board following instructions on the MicroPython website. Then use a tool such as rshell to upload the following files to the board:

main.py
config.py
index.html
microdot.py

You can find microdot.py in the src/microdot directory of this repository.

If you are using a low end microcontroller such as the ESP8266, it is quite possible that the microdot.py file will fail to compile due to the MicroPython compiler needing more RAM than available in the device. In that case, you can install the mpy-cross Python package in your computer (same version as your MicroPython firmware) and precompile this file. The precompiled file will have the name microdot.mpy. Upload this file and remove microdot.py from the device.

When the device is restarted after the files were uploaded, it will connect to Wi-Fi and then start a web server on port 8000. One way to find out which IP address was assigned to your device is to check your Wi-Fi's router administration panel. Another option is to connect to the MicroPython REPL with rshell or any other tool that you like, and then press Ctrl-D at the MicroPython prompt to soft boot the device. The IP address is printed to the terminal on startup.

You should not upload other .py files that exist in this directory to your device. These files are used when running with emulated hardware.

Trying out the application

Once the device is running the server, you can connect to it using a web browser. For example, if your device's Wi-Fi connection was assigned the IP address 192.168.0.145, type http://192.168.0.45:8000/ in your browser's address bar. Note it is http:// and not https://. This example does not use the TLS/SSL protocol.

To test the JSON API, you can use curl or your favorite HTTP client. The API endpoint uses the /api path, with the same URL as the main website. Here is an example using curl:

$ curl http://192.168.0.145:8000/api
{"temperature": 21.6, "humidity": 58.9, "time": 1752444652}

The temperature value is given in degrees Celsius. The humidity value is given as a percentage. The time value is a UNIX timestamp.

Running in Emulation mode

You can run this application on your computer, directly from this directory. When used in this way, the DHT22 hardware is emulated, and the temperature and humidity values are randomly generated.

The only dependency that is needed for this application to run in emulation mode is microdot, so make sure that is installed, or else add a copy of the microdot.py from the src/microdot directory in this folder.