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	×	Micro USB Cable
1	×	Capacitive Soil Moisture Sensor
1	×	Jumper Wires
1	×	(Optional) 5V Power Adapter
1	×	(Optional) DC Power Jack
1	×	(Optional) ESP32 Screw Terminal Adapter

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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 correction on a resistive soil moisture sensor.

resistive soil moisture sensor correction

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 direct 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 higher the voltage in the AOUT pin is

Wiring Diagram

This image is created using Fritzing. Click to enlarge image

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 GIOP36 (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
Plant the moisture sensor into the soil
Irrigate the soil slowly
See the result on Serial Monitor. It looks like the below:

COM6

Send

Moisture value: 32 Moisture value: 116 Moisture value: 203 Moisture value: 403 Moisture value: 803 Moisture value: 1284 Moisture value: 2291 Moisture value: 2319

Autoscroll Show timestamp

Clear output

9600 baud

Newline

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 GIOP36 (ADC0) that connects to AOUT pin of moisture sensor #define THRESHOLD 1000 // 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 (31) The soil is DRY (458) The soil is DRY (716) The soil is DRY (734) The soil is DRY (889) The soil is WET (1158) The soil is WET (1292) The soil is WET (2456) The soil is WET (2961)

Autoscroll Show timestamp

Clear output

9600 baud

Newline

Video Tutorial

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Function References

analogRead()

Learn More

ESP32 - Automatic Irrigation System

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