Code

You will code your moisture meter using a pot of dry dirt and wet dirt. This is so you can set the micro:bit to know what both dry and wet conditions are.

Step 1: Measuring moisture

The soil itself has some electrical resistance which depends on the amount of water and nutrients in it. It acts like a variable resistor in an electronic circuit. The water is not conductive but the nutrient content is. The combination of water and soil nutrients makes the soil have some conductivity. So, the more water there is, combined with the nutrients, the less the soil will have electrical resistance.

To measure this, we read the voltage on pin P0 using ||pins:analog read pin|| which returns a value between 0 (no current) and 1023 (maximum current). The value is graph on the screen using ||led:plot bar graph||.

basic.forever(() => {
    led.plotBarGraph(
        pins.analogReadPin(AnalogPin.P0),
        1023
    )
})

Experiment!

• Insert the nails in the dry dirt and you should see most LEDs turn off.
• Insert the nail in the wet dirt and you should see most LEDs turn on.

Step 2: Sensor data values

In the previous program, we only have a rough idea of what the sensor value is. It’s using just a tiny screen to display it! Let’s add code that displays the current reading when button A is pressed.

This code needs to go into the ||basic:forever|| loop. We’ve also added the variable reading to store the reading value.

let reading = 0
basic.forever(() => {
    reading = pins.analogReadPin(AnalogPin.P0)
    led.plotBarGraph(
        reading,
        1023
    )
    if (input.buttonIsPressed(Button.A)) {
        basic.showNumber(reading)
    }
})

Experiment!

Test and record the P0 input values for both very dry dirt and for dirt that is wet. This will let you know the what the moisture scale of your meter is. The dry soil will have a low value and the wet soil will have a higher value.

1. Insert the nails in the dry dirt, press A and record the value.
2. Insert the nails in the wet dirt, press A and record the value.

Here’s an example test table of results for very dry and wet dirt using both versions of the micro:bit:

Step 3: Don’t waste energy!

We want our soil probes to work for a long time and to save our battery power, so we need to tweak our code so our moisture sensor doesn’t use too much energy.

• Our circuit connects directly to the 3V pin so it is always using electricity. Instead, we will connect it to P1 and turn that pin high only while the measurement is taken. This saves electricity and also avoids corrosion of the probes.
• We will also lower the brightness of the screen to lower the energy consumption from the LEDs.
• Soil moisture changes very slowly so we don’t need to measure it all the time!!! Let’s add a sleep of 5 seconds in the loop as well.

led.setBrightness(64)
let reading = 0
basic.forever(() => {
    pins.analogWritePin(AnalogPin.P1, 1023)
    reading = pins.analogReadPin(AnalogPin.P0)
    pins.analogWritePin(AnalogPin.P1, 0)
    led.plotBarGraph(
        reading,
        1023
    )
    if (input.buttonIsPressed(Button.A)) {
        basic.showNumber(reading)
    }
    basic.pause(5000);
})

Experiment!

• Using the dry soil and wet soil pots, test that your circuit still works. Remember you’ll have to wait up to 10 seconds to see a change!