Use Python to calibrate your Kitronik Prong Moisture Sensor

BBC microbit Kitronik MicroPython moisture sensor mu editor Thonny

Picture of Kitronik Prong soil moisture sensor and plant

 In this tutorial, you will learn to:

  • Use Python to read the output from a moisture sensor
  • Use mu or Thonny editors to debug your micro:bit code
  • Calibrate your code to detect the right amount of moisture

You will need 

  • One Kitronik Prong moisture sensor, assembled with a micro:bit
  • A pot plant that has not been watered on the same day
  • A micro USB cable to program your micro:bit
  • A battery pack (2 x AAA batteries) for the micro:bit
  • Mu editor installed on your computer [1]

Note: if you are using a Raspberry Pi to program your micro:bit, you can also use the installed Thonny editor.

Step 1: Launch an editor, and enable micro:bit mode

Mu editor
  • Connect your micro:bit to your computer with the micro USB cable
  • Launch mu editor
  • Click on the mode mu editor mode icon button in the bottom right of the editor
  • Select BBC micro:bit and click OK
    set mode in mu editor
Thonny editor
  • Connect your micro:bit to your computer with the micro USB cable
  • Launch Thonny editor Thonny icon
  • Click on the Thonny menu and select Preferences
  • Go to the Interpreter tab and select "MicroPython (BBC micro:bit) then click on OK
    set mode in Thonny editor

Step 2: Write your code

  • Create a new file in your editor by clicking on new mu editor new icon in mu or File > New in Thonny
  • First we need to add a line to import the micro:bit functions:
    from microbit import *
  • Our prong moisture sensor is connected to pin1 of the micro:bit. Add a while True: loop to repeatedly store the micro:bit pin1 value in the moisture variable: 
    from microbit import *
    while True:
        moisture = pin1.read_analog()
  • Add a print() command to send the moisture value to our editor’s debug console, then use a sleep() command to wait for 1000 milliseconds before repeating:
    from microbit import *
    while True:
        moisture = pin1.read_analog()

Step 3: Save and run your code

We can save our code to our computer or send it to the micro:bit. It's a good idea to save a copy of our code to the computer first, and then test it on the micro:bit.

On Thonny, when you click on the FileSave As menu item, you can choose whether to save to your computer or your micro:bit. When you save to your micro:bit, overwrite the file, main.pyThonny overwrite
  • On mu editor, the Save button lets you save to a location on your computer, but the Flash button is used to save to the micro:bit
  • Once the code is saved on the micro:bit it will restart the micro:bit and run it automatically.
    On mu editor, you can click the REPL mu editor REPL icon button so you can see the moisture information from the print() command.
mu editor REPL
On Thonny, the moisture information will appear in the Shell window below.thonny shell
  • To stop your code running, when using mu, hold down the control key and press c (ctrl-c); or on Thonny, you can click the Stop button.

    Step 4: Calibrate your code

    Using the shell/REPL, we can view the moisture level as we add water to our plant, and write our code to accurately react to that calibrated moisture level.

    • Place the lower part of the prong into the soil of your plant and run your code. Make a note of the number that comes up in the dry soil
    • Slowly add water until the soil is moist but not so full that there are puddles. Make a note of the number for the wet soil.
    • Update your code to do something when the moisture level is too low, and something else when it is wet enough.
      Hint: use an if command right after the sleep(1000) command as shown in the example below, and insert the number you recorded earlier for the wet soil.
      from microbit import *
      while True:
          moisture = pin1.read_analog()
          if moisture > 400:

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