Smart Plant Care! Build a Qwiic Soil Moisture Sensor System

2025-03-25 | By SparkFun Electronics

License: See Original Project Qwiic

Guide by Chowdah

Introduction

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Have you ever wanted your plants to tell you how they're feeling? While we might be a ways ‎away from understanding or communicating with plant consciousness, we can tell if they're ‎thirsty or not, and with the SparkFun Soil Moisture Sensor, you can do just that! This tutorial ‎will show you how to take soil moisture readings and then display those readings on an ‎OLED as a happy, meh, or sad face, depending on how wet or dry the soil is.‎

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Materials

Required Materials

To follow along with the project at the end of this tutorial, you will need the following. You ‎may not need everything, though, depending on what you already have and which sensor ‎you prefer. Add it all to your cart and modify, as necessary.‎

Note: The 9V battery isn't suitable to keep plugged in for long periods of time. With this ‎current configuration, the project will last plugged in for about a day. We recommend only ‎powering the project when you want to see how your plant is feeling!‎

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Hardware Setup

The hardware setup for this project is super straightforward. Attach the Qwiic Soil Moisture ‎Sensor to the Qwiic OLED via a Qwiic Cable and then attach the OLED to the RedBoard. We ‎used a 9V battery and 9V battery holder to power this project so that we could keep it ‎portable, but you could use a portable charger or hook it up straight to your computer ‎depending on your use case and availability of parts on hand. We also used some screws ‎and a 3D-printed base plate to keep the display and sensor together, but that is also not ‎necessary. So simple!

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Calibration

System Calibration To get any sort of useful data out of your Soil Moisture Sensor, it is ‎advised that you calibrate it to whatever soil you plan to monitor. Different types of soil can ‎affect the sensor, and you may get different readings from one composition to the next. ‎Before you start storing moisture data or triggering events based on that value, you should ‎see what values you are actually getting from your sensor. Using the sketch above, note ‎what values your sensor outputs when the sensor is completely dry vs when the sensor is ‎completely submerged in a shallow cup of water. Depending on what microcontroller you're ‎using, the operating voltage of that microcontroller, and the resolution of its analog-to-‎digital converter, your results will vary.‎

For example, using the same circuit above, if I detach the VCC pin from D7 and attach it ‎directly to the 5V supply on the RedBoard, you'll see the close to the following values in the ‎serial monitor when the sensor is dry (~0) and when it is completely saturated with ‎moisture (~880).‎

5V_Cal

Code

The code for this project is really simple, just download the Arduino libraries for the ‎products used and past the code below into the terminal! Make sure to adjust the moisture ‎values to suit your plant's needs. The values used below seem to work pretty well for my ‎money tree, which is a pretty low-fuss plant.‎

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#include <Wire.h>
#include <SparkFun_Qwiic_OLED.h>
#include <res/qw_fnt_5x7.h>

QwiicMicroOLED myOLED;

#define COMMAND_GET_VALUE   0x05
const uint8_t qwiicAddress = 0x28;
uint16_t ADC_VALUE = 0;


const int dryValue = 1023;
const int wetValue = 400;

void setup() {
    Serial.begin(115200);
    Wire.begin();

    while (myOLED.begin() == false) {
        Serial.println("OLED not connected, check wiring!");
        delay(1000);
    }

    testForConnectivity();
}

void loop() {
    int moistureValue = getMoisture();
    displayMoisture(moistureValue);
    delay(1000);
}

int getMoisture() {
    get_value();
    return map(ADC_VALUE, dryValue, wetValue, 0, 100);
}

void get_value() {
    Wire.beginTransmission(qwiicAddress);
    Wire.write(COMMAND_GET_VALUE);
    Wire.endTransmission();

    Wire.requestFrom((uint8_t)qwiicAddress, (uint8_t)2);

    if (Wire.available() >= 2) {
        uint8_t ADC_VALUE_L = Wire.read();
        uint8_t ADC_VALUE_H = Wire.read();
        ADC_VALUE = (ADC_VALUE_H << 8) | ADC_VALUE_L;
    }
}

void displayMoisture(int moistureValue) {
    myOLED.erase();

    if (moistureValue < 30) {
        // Happy Face when well-watered
        myOLED.line(20, 0, 30, 0);   // Left eye
        myOLED.line(40, 0, 50, 0);   // Right eye

        // Smile: \__/
        myOLED.line(20, 20, 30, 10);   // Left smile
        myOLED.line(30, 10, 40, 10);   // Bottom of smile
        myOLED.line(40, 10, 50, 20);   // Right smile
    } else if (moistureValue >= 50 && moistureValue < 70) {
        // Neutral Face
        myOLED.line(20, 0, 30, 0);   // Left eye
        myOLED.line(40, 0, 50, 0);   // Right eye
        myOLED.line(20, 20, 50, 20);   // Neutral line
    } else {
        // Sad Face when dry
        myOLED.line(20, 0, 30, 0);   // Left eye
        myOLED.line(40, 0, 50, 0);   // Right eye

        // Sad mouth: /--\
        myOLED.line(20, 10, 30, 20);   // Left sad
        myOLED.line(30, 20, 40, 20);   // Bottom of sad
        myOLED.line(40, 20, 50, 10);   // Right sad
    }

    myOLED.display();

    Serial.print("Moisture: ");
    Serial.print(moistureValue);
    Serial.println("%");
}

void testForConnectivity() {
    Wire.beginTransmission(qwiicAddress);
    if (Wire.endTransmission() != 0) {
        Serial.println("Check connections. No soil sensor found.");
        while (1);
    }
}