Build a Smart Irrigation System Using Soil Moisture Sensor

You build a smart irrigation system by connecting a soil moisture sensor to a microcontroller and a relay-controlled water pump. The sensor detects soil dryness, triggering the microcontroller to run the pump until the soil is moist again.

Medium ⏱ 45 - 60 minutes 🔭 Class 6-8 🏠 4 of 9 things are at home 🧑‍🔬 Do it with an adult

Time: 45 - 60 minutes
Difficulty: Medium
Best for: Class 6-8
Subject: Environmental Science
Things needed: 9, 4 at home
Steps: 13
Cost: Low, a few things to find
You will learn: Soil Moisture Sensing and Automated Water Conservation

Build a Smart Irrigation System Using Soil Moisture Sensor, Experiment · Class 6-8 | jijnas.com

Before you start

What you need

0 of 9 ready

Tap each thing once you have it in front of you. Items with the house sign are already somewhere in your home.

Capacitive or resistive soil moisture sensor module 1 unit No luck? Use two stainless steel nails inserted in soil connected to a transistor switch.
Arduino Uno microcontroller board 1 unit
5V single-channel relay module 1 unit No luck? Use nPN transistor like BC547 with a diode if using a small DC motor.
5V or 12V DC mini submersible water pump 1 unit No luck? Use small 3V-6V DC hobby motor with a plastic fan blade to demonstrate motor triggering.
🏠 Flexible thin plastic tubing (pipe) 1 meter No luck? Use used aquarium air tube or clear plastic straw link.
Jumper wires (Male-to-Female, Male-to-Male) 10-12 wires No luck? Use thin insulated copper wire stripped at ends.
🏠 Plastic container or bottle (for water reservoir) 1 container No luck? Use cut plastic cold drink bottle or plastic mug.
🏠 Small plant pot filled with dry soil 1 pot No luck? Use plastic tea cup filled with garden soil.
🏠 9V battery with connector or 5V DC power adapter 1 unit No luck? Use power bank with a USB cable.

💬 Send to a parent

Think first

What do you think will happen?

Real scientists guess before they look. Write one line now. After the steps, the page will show it next to what you should see.

Safety first, an adult should be with you.

Electrical hazard: Ensure only low-voltage DC components (5V to 12V) are used for the pump and relay. Do not connect mains AC voltage.
Water and electronics hazard: Keep the microcontroller, relay, and power source isolated and shielded from water splashing or leakage.

Do it

The procedure

Tap a step when it is done. Lab mode shows one step at a time in big letters, reads it aloud if you want, and keeps the timer.

Position your plant pot and water reservoir on a flat surface, keeping all electronic boards and wire connections safely away from potential water splashes.Keep all control electronics, power sources, and relays strictly isolated from water leakage or splashing to prevent electrical short circuits.
Insert the soil moisture sensor vertically into the root zone of your plant pot, ensuring the soil fits snugly against the probe without air gaps.
Attach one end of the plastic tubing to the output nozzle of the submersible DC water pump.
Place the DC water pump into the water container, directing the free end of the tubing toward the top of the plant pot.
Connect the soil moisture sensor output pins to the designated analog pin, power pin, and ground pin on your microcontroller using jumper wires.
Wire the control pin of the relay module to a digital pin on the microcontroller, then connect its power and ground pins.
Connect the low-voltage DC water pump wires through the relay switch contacts and connect them to your DC power source.Use only low-voltage DC (5V to 12V) power for the pump and relay. Never connect mains AC power to this project.
Write a script that reads analog moisture values, compares them against a chosen dry soil threshold, and triggers the relay pin HIGH when the soil is dry.
Include code instructions to set the relay pin LOW to switch off the pump once moisture exceeds the threshold.
Upload the written control program to your microcontroller using a USB cable.
Power the microcontroller using your low-voltage battery or DC adapter to start automatic operation.Ensure all power inputs match low-voltage DC ratings (5V-12V) before plugging in.
Saturate the soil around the probe fully with water to complete initial calibration and confirm the baseline setpoint.
Monitor the setup to verify that the pump activates when the soil dries out and automatically stops after delivering water.

01 · The idea

The Concept

Automated irrigation uses electronic feedback loops to optimize water delivery to crops or household plants. By reading environmental conditions directly from the soil, the system replaces rigid mechanical timers and eliminates manual watering errors.

02 · What you should see

Observation

When you insert the sensor into dry soil, the water pump turns on automatically and starts pumping water into the soil container. As the soil gets wet and absorbs water, the system senses the moisture and turns off the pump on its own.

If you add an optional LCD screen, it displays Soil: DRY | Pump: ON when the soil needs water, and switches to Soil: WET | Pump: OFF as soon as the plant gets enough water.

03 · Why it happens

Why It Happens

Think of the soil moisture sensor as a hungry belly monitor for your plant. When the soil becomes dry, it sends an electrical signal to the microcontroller (the brain of the project), telling it that the plant is thirsty.

The microcontroller then flips an electronic switch called a relay. The relay opens the power line to the water pump, turning it on just like switching on a tap. Once the soil absorbs enough water, the sensor signals that the plant is full, and the brain turns off the pump to prevent overwatering.

04 · The science

The Science Behind It

The system relies on detecting the volumetric water content of soil through electrical conductivity or dielectric capacitance. Water is a good conductor of electricity compared to dry dirt, so wet soil allows electrical current to flow much more easily than dry soil. The soil moisture sensor measures this change and sends an analog voltage signal to the microcontroller.

The microcontroller compares this incoming reading to a pre-programmed threshold value. If the reading drops below the setpoint, it triggers the relay module to complete the circuit for the water pump. Just like in the NCERT chapter What If It Finishes?, which highlights the need to conserve natural resources before they run out, smart irrigation saves precious freshwater by delivering water only when needed and eliminating unnecessary waste.

05 · Take it further

Make It a Project

If you are presenting this automated system at a science fair, try altering single parameters to evaluate performance:

Independent Variable: Soil moisture sensor threshold setting or soil type (sandy, loam, clay).
Dependent Variable: Total volume of water used (mL), duration of watering cycles (seconds), and soil moisture retention over time.
Controlled Variables: Plant type, ambient room temperature, light exposure, initial soil dry weight, and pump flow rate.

Record baseline sensor readings in completely dry soil and fully saturated soil for calibration. Measure the exact volume of water delivered during each cycle to calculate total water savings compared to a fixed timer.

06 · Questions students ask

FAQs

Why does my water pump stay on all the time?

Check your threshold value in the code. If your setpoint is too high, the system thinks the soil is always dry. Also make sure your sensor probes are firmly buried in the soil and your relay wiring is connected to the Normally Open (NO) terminal.

Can I use a 9V battery to power the water pump?

Standard 9V transistor batteries run out of power very quickly under motor loads. Use a 5V-12V power adapter or a rechargeable Li-ion battery pack instead so your pump gets enough current to move water without crashing your microcontroller.

What is the difference between resistive and capacitive moisture sensors?

Resistive sensors pass electricity directly through the soil, causing the metal probes to corrode quickly. Capacitive sensors measure moisture without exposed metal, making them rust-proof, longer-lasting, and far more reliable for long-term science projects.

How do I set the right threshold value for my soil?

Test your sensor readings in completely dry soil, then in saturated wet soil using your Serial Monitor. Pick a number halfway between these two readings and set it as your threshold in the microcontroller code.

Why is a relay module necessary instead of connecting the pump directly to Arduino?

Microcontroller pins can only deliver a tiny amount of current (around 20mA to 40mA), which is not enough to run a motor. A relay acts as an electrically controlled switch that allows a small signal from Arduino to safely control a larger power supply.

Write it up

Your lab notebook

Two minutes here and your lab record is done. Everything stays on this device until you copy it out.

Did it work?

What I actually saw What I would change next time

💼 Same science, as a real job

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