SENSORS/ACTUATORS

INTERNET OF THING (IoT)

ESP32 - Soil Moisture Sensor

This tutorial instructs you how to use ESP32 to read the soil moisture from sensor. In detail, we will learn:

Capacitive Moisture Sensor vs Resistive Moisture Sensor
How to use ESP32 to read the value from a capacitive moisture sensor
How to calibrate a capacitive moisture sensor
How ESP32 determines if the soil is wet or dry

Hardware Used In This Tutorial

1	×	ESP-WROOM-32 Dev Module
1	×	USB Cable Type-C
1	×	Capacitive Soil Moisture Sensor
1	×	Breadboard
1	×	Jumper Wires
1	×	(Optional) DC Power Jack
1	×	(Recommended) ESP32 Screw Terminal Adapter

Or you can buy the following sensor kits:

1	×	DIYables Sensor Kit (30 sensors/displays)
1	×	DIYables Sensor Kit (18 sensors/displays)

Disclosure: some of these links are affiliate links. We may earn a commission on your purchase at no extra cost to you. We appreciate it.

Buy Note: Numerous capacitive soil moisture sensors available in the market are of poor quality, regardless of the version. We strongly advise purchasing the sensor from the DIYables brand via the link above; we conducted tests, and it performed reliably.

Introduction to Soil Moisture Sensor Sensor

capacitive moisture sensor vs resistive moisture sensor

There are two types of moisture sensors:

Resistive moisture sensor
Rapacitive moisture sensor.

Both sensors output the soil moisture value. However, their working principles are different. We highly recommend using the capacitive moisture sensor, because of the following reasons:

The resistive soil moisture sensor corrodes over time. The electrical current that flows between the sensor's probes causes electrochemical corrosion.
The capacitive soil moisture sensor does NOT corrode over time. The sensor's electrodes are not exposed and no electrical current folows between them.

The below image shows the corrosion on a resistive soil moisture sensor.

resistive soil moisture sensor corrosion

The rest of this tutorial uses the capacitive soil moisture sensor.

Capacitive Soil Moisture Sensor Pinout

A capacitive soil moisture sensor has three pins:

GND pin: connect this pin to GND (0V)
VCC pin: connect this pin to VCC (5V or 3.3v)
AOUT pin: Analog signal output pin outputs the voltage in inverse proportion to the soil moisture level. Connect this pin to an ESP32's analog input pin.

How It Works

The more the water presents in the soil, the lower the voltage in the AOUT pin is

Wiring Diagram

This image is created using Fritzing. Click to enlarge image

If you're unfamiliar with how to supply power to the ESP32 and other components, you can find guidance in the following tutorial: How to Power ESP32.

ESP32 Code

/* * This ESP32 code is created by esp32io.com * * This ESP32 code is released in the public domain * * For more detail (instruction and wiring diagram), visit https://esp32io.com/tutorials/esp32-soil-moisture-sensor */ #define AOUT_PIN 36 // ESP32 pin GPIO36 (ADC0) that connects to AOUT pin of moisture sensor void setup() { Serial.begin(9600); } void loop() { int value = analogRead(AOUT_PIN); // read the analog value from sensor Serial.print("Moisture value: "); Serial.println(value); delay(500); }

Quick Instructions

Copy the above code and paste it to Arduino IDE
Click Upload button on Arduino IDE to upload code to ESP32 board
Bury the sensor in soil, then pour water into the soil. Or slowly submerge it into a cup of salt water.
Check out the result on Serial Monitor. It looks like the below:

COM6

Send

Moisture value: 3126 Moisture value: 3031 Moisture value: 2937 Moisture value: 2880 Moisture value: 2790 Moisture value: 2719 Moisture value: 2655 Moisture value: 2606 Moisture value: 2640 Moisture value: 2631 Moisture value: 2640 Moisture value: 2624

Autoscroll Show timestamp

Clear output

9600 baud

Newline

※ NOTE THAT:

Avoid using pure water for testing, as it doesn't conduct electricity, thus won't affect sensor readings.
The sensor readings never reach zero. It's normal for the values to range between 3100 and 2600, although this may vary depending on some factors: the depth of sensor embedding, the composition of the soil or water, voltage of power supply.
Do not bury the circuit part (located on top of the sensor) in soil or water, as this could potentially damage the sensor.

Calibration for Capacitive Soil Moisture Sensor

The measured value from the moisture sensor is relative. It depends on the soil's composition and water. In practice, we need to do calibration to determine a threshold that is a border between wet and dry.

How to do calibration:

Run the above code on ESP32
Plant the moisture sensor into the soil
Plant the moisture sensor into the soil
Irrigate the soil slowly
Watch Serial Monitor.
Write down a value at the time you feel that the soil changes its moisture from dry to wet. This value is called THRESHOLD.

Determine if the soil is wet or dry

After the calibration, update the THRESHOLD value you wrote down to the below code. The below code determines if the soil is wet or dry

/* * This ESP32 code is created by esp32io.com * * This ESP32 code is released in the public domain * * For more detail (instruction and wiring diagram), visit https://esp32io.com/tutorials/esp32-soil-moisture-sensor */ #define AOUT_PIN 36 // ESP32 pin GPIO36 (ADC0) that connects to AOUT pin of moisture sensor #define THRESHOLD 2800 // CHANGE YOUR THRESHOLD HERE void setup() { Serial.begin(9600); } void loop() { int value = analogRead(AOUT_PIN); // read the analog value from sensor if (value > THRESHOLD) Serial.print("The soil is DRY ("); else Serial.print("The soil is WET ("); Serial.print(value); Serial.println(")"); delay(500); }

The result on Serial Monitor.

COM6

Send

The soil is DRY (3126) The soil is DRY (3041) The soil is DRY (2967) The soil is DRY (2840) The soil is WET (2791) The soil is WET (2719) The soil is WET (2657) The soil is WET (2643) The soil is WET (2624)

Autoscroll Show timestamp

Clear output

9600 baud

Newline

Video Tutorial

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Function References

analogRead()

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