Use Python to calibrate your Kitronik Prong Moisture Sensor

 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  button in the bottom right of the editor
• Select BBC micro:bit and click OK
  
  

Thonny editor

• Connect your micro:bit to your computer with the micro USB cable
• Launch Thonny editor 
• Click on the Thonny menu and select Preferences
• Go to the Interpreter tab and select "MicroPython (BBC micro:bit) then click on OK
  
  

Step 2: Write your code

• Create a new file in your editor by clicking on new  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()
      print(moisture)
      sleep(1000)
  

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 File > Save 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.py

• 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  button so you can see the moisture information from the print() command.

• 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()
      print(moisture)
      sleep(1000)
      if moisture > 400:
           display.show(Image.HAPPY)
      else:
           display.show(Image.SAD)