pxt-calliope/docs/projects/soil-moisture/code.md

Code

You will need to get a pot of dry dirt and wet dirt for the coding.

Step 1: Measuring moisture

The soil acts as a variable resistor based on the amount of water and nutrient. The water is not conductive but the nutrient are, so more water, more nutrient, so the soil will be less resistive.

To measure this, we read the voltage on pin P0 using analog read pin which returns a value between 0 (no current) and 1023 (max 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 off
• insert the nail in the wet dirt and you should see most LEDs on

Step 2: Sensor data values

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

This code has to be inserted in the forever loop. We've also added a 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!

• insert the nails in the dry dirt, press A and note the value. You should see a value close to 250 for dry dirt.
• insert the nails in the wet dirt, press A and note the value. You should see a value close to 1000 for wet dirt.

https://youtu.be/S8NppVT_paw

Step 3: Don't waste energy!

We want our probe to work for a long time on batteries so we need to tweak our code to save 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 electricty 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 all the times!!! 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 and wet dirt pots, test that your ciruit still works. Remember you'll have to wait up to 10 seconds to see a change!

~button /projects/soil-moisture/connect

Connect

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