SENSORS/ACTUATORS

Arduino Nano - Soil Moisture Sensor

This tutorial instructs you how to use a moisture sensor with Arduino Nano. Specifically, we will look at:

The differences between a resistive and capacitive moisture sensor
How to program Arduino Nano to read the value of the moisture sensor using Arduino Nano
How to use Arduino Nano to calibrate the moisture sensor
How Arduino Nano determines if the soil is wet or dry

Hardware Preparation

1	×	Arduino Nano
1	×	USB A to Mini-B USB cable
1	×	Capacitive Soil Moisture Sensor
1	×	Jumper Wires
1	×	(Optional) 9V Power Adapter for Arduino Nano
1	×	(Recommended) Screw Terminal Adapter for Arduino Nano

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 the links provided in this section are Amazon affiliate links. We may receive a commission for any purchases made through these links at no additional cost to you. We appreciate your support.

Buy Note: Many soil moisture sensors available in the market are unreliable, regardless of their version. We strongly recommend buying the sensor from the DIYables brand using the link provided above. We tested it, and it worked reliably.

Overview of Soil Moisture Sensor Sensor

capacitive moisture sensor vs resistive moisture sensor

There are two types of moisture sensors:

The resistive moisture sensor
The capacitive moisture sensor.

Both sensors offer soil moisture information. However, their working principles are different. We strongly suggest utilizing the capacitive moisture sensor due to the following:

The resistive soil moisture sensor is prone to corrosion over time. This is because electrical current passes between its probes, leading to electrochemical corrosion.
The capacitive soil moisture sensor corrodes with time much slower than the resistive soil moisture sensor. This is because its electrodes are not exposed and are comparatively more resistant to corrosion.

This is an illustration of a resistive soil moisture sensor that has been damaged due to corrosion over time.

The remainder of this tutorial will utilize the capacitive soil moisture sensor.

Capacitive Soil Moisture Sensor Pinout

The capacitive soil moisture sensor has three pins:

GND pin: This should be connected to the GND (0V)
VCC pin: This should be connected to VCC (5V or 3.3v)
AOUT pin: This analog signal output pin will produce a voltage that is proportional to the soil moisture level. This should be connected to an Arduino's analog input pin.

How It Works

The AOUT pin has a higher voltage when there is less water in the soil.

Wiring Diagram

This image is created using Fritzing. Click to enlarge image

Arduino Nano Code for reading value from soil moisture sensor

/* * This Arduino Nano code was developed by newbiely.com * * This Arduino Nano code is made available for public use without any restriction * * For comprehensive instructions and wiring diagrams, please visit: * https://newbiely.com/tutorials/arduino-nano/arduino-nano-soil-moisture-sensor */ #define AOUT_PIN A6 // The Arduino Nano pin 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: "); Serial.println(value); delay(500); }

Detailed Instructions

Copy the code above and open it with the Arduino IDE.
Click the Upload button on the Arduino IDE to compile and upload the code to the Arduino Nano.
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: 581 Moisture: 581 Moisture: 575 Moisture: 566 Moisture: 556 Moisture: 547 Moisture: 539 Moisture: 530 Moisture: 521 Moisture: 513 Moisture: 506 Moisture: 500 Moisture: 495 Moisture: 492 Moisture: 490 Moisture: 489 Moisture: 488

Autoscroll Show timestamp

Clear output

9600 baud

Newline

※ NOTE THAT:

Avoid testing with pure water as it doesn't conduct electricity, which won't affect the sensor readings.
The sensor readings won't ever hit zero. Typically, they range between 500 and 600, although this can vary depending on factors like the sensor's depth, soil or water type, and power supply voltage.
Do not bury the circuit part (located on top of the sensor) in soil or water, as this may damage the sensor.

Calibration for Capacitive Soil Moisture Sensor

The value obtained from the moisture sensor is not absolute. It depends on the soil's composition and amount of water present. In order to accurately determine a boundary between wet and dry, calibration must be done.

Instructions for calibration:

Execute the code on Arduino Nano
Place the moisture sensor in the soil
Gradually add water to the soil
Check out the Serial Monitor
Record the value when the soil changes from dry to wet. This is referred to as the THRESHOLD.

Arduino Nano determines if the soil is wet or dry

Once the calibration is complete, update the THRESHOLD value that you noted down into the code below. This Arduino Nano code will decide whether the soil is wet or dry.

/* * This Arduino Nano code was developed by newbiely.com * * This Arduino Nano code is made available for public use without any restriction * * For comprehensive instructions and wiring diagrams, please visit: * https://newbiely.com/tutorials/arduino-nano/arduino-nano-soil-moisture-sensor */ #define AOUT_PIN A6 // The Arduino Nano pin that connects to AOUT pin of moisture sensor #define THRESHOLD 530 // 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 output displayed on the Serial Monitor.

COM6

Send

The soil is DRY (581) The soil is DRY (575) The soil is DRY (566) The soil is DRY (547) The soil is DRY (539) The soil is WET (521) The soil is WET (513) The soil is WET (492) The soil is WET (488)

Autoscroll Show timestamp

Clear output

9600 baud

Newline

Video Tutorial

Function References

Learn More

Arduino Nano - Irrigation

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