Soil Moisture Sensor for micro:bit

by richard | Aug 23, 2017 | 0 comments

This project uses a moisture sensor to create a simple device to warn when soil becomes too dry.

Language: MicroPython

Concepts: displaying images; using digital & analogue pins

Requires:

BBC micro:bit
YL-38 YL-69 moisture sensor
Connecting wires

Optional:

Prototyping breadboard

Step by Step Instructions

Step 1

Connect the two pins on the YL-69 probe to the connectors at the top of the YL-38 module board.

Notes

It doesn’t matter which way you make these connections.

Step 2

Connect GND (0V) on the micro:bit to GND on the YL-38
Connect PIN 2 on the micro:bit to Vcc on the YL-38
Connect PIN 3 on the micro:bit to A0 on the YL-38

Notes

PIN 2 is used to turn on the circuit so that a reading can be taken.

PIN 3 is used to read the analogue signal from the sensor. This is a voltage that is high when the moisture content is low and lower when the moisture content is high.

Step 3

Using your preferred editor, type or copy/paste the code and flash to the micro:bit.

Code

from microbit import *

# sensor reading above which the soil needs watering
threshold = 500

# time in milliseconds between readings
interval = 30000

while True:
    # turn on sensor (pin 2 connected to Vcc on sensor)
    pin2.write_digital(1)

    # wait for sensor to settle
    sleep(5000)

    # read analog pin 3 (connected to A0 on sensor)
    moist = pin3.read_analog()

    # print reading to console
    print(str(moist))

    # needs watering
    if moist > threshold:
        # show sad face
        display.show(Image.SAD, clear=False)
    else:
        # show happy face
        display.show(Image.HAPPY, clear=False)

    # turn off sensor and sleep to conserve sensor contacts
    pin2.write_digital(0)
    sleep(interval)

# end

Notes

In our example we have used both a breakout board for the micro:bit and a prototyping breadboard to make the connections. In theory you could connect the sensor directly to the micro:bit using crocodile clips but this is likely to be unreliable in practice.

Further Development

Add code to query the last reading taken when button A on the micro:bit is pressed.
Store the last 5 readings and present these as a bar chart using the LEDs on the micro:bit when button B is pressed.