Learning Objective
Understand how a capacitive soil sensor notices water in soil, calibrate it for dry and wet, and use it to build a "Thirsty Plant" detector for Smart Agriculture.
Smart Agriculture (Soil Moisture)
Hardware: Capacitive Soil Moisture Sensor
Dashboard: Soil Moisture Percentage Card
See How the Soil Moisture Sensor Works
Look at the kit's sensor, then read the three cards in order: the change in the world → the sensor signal → a useful action.
1 · What it notices
How much water is in the soil around its blade.
2 · How it works
Two copper areas under the coating hold more charge when water is near. MakerBuddy turns that into a 0–100 % level using your Dry and Wet calibration.
3 · Why it matters
Too little or too much water harms plants, so checking helps you water on time.
Where might we use it?
- Houseplant reminders
- Greenhouse watering
- Farming that saves water
Student sentence: A capacitive soil sensor notices water by how much electric charge its coated blade can hold.
Teacher/parent prompt: Ask, “What changes in the real world, what does the sensor turn it into, and what could happen next?”
Theory: Capacitive Sensing & Soil Moisture
From Sensing Air to Sensing Earth
Now you will sense soil instead of air. This sensor doesn't measure water directly. Instead, it notices how much water is close to its blade, because water changes how much electric charge the blade can hold (its capacitance).
How a Capacitive Sensor Works
The kit's sensor is a flat, coated blade. No bare metal touches the soil, so it does not rust like older two-prong probes. Under the coating, it charges and empties two copper areas very fast. Water helps them hold far more charge than air or dry soil does. The sensor turns that charge into a voltage. On this sensor, wetter soil gives a LOWER raw number.
Understanding the Sensor Reading
MakerBuddy reads the voltage as a raw number (0–4095) and turns it into 0–100% using your calibration. 0% means 'as dry as my Dry setting' and 100% means 'as wet as my Wet setting'. Once calibrated, roughly:
- Dry soil: about 0–30%
- Moist soil: about 30–60% (comfortable for most plants)
- Wet soil: about 60–100%
- Water up to the line: 100%
Remember: wetter soil, lower raw number.
Calibrating the Moisture Percentage
Some sensors give higher raw numbers when wet, others lower. So calibrate both ends yourself, never with someone else's numbers.
- On the Soil Moisture card, click the ⚙️ button (or the card itself) to open Soil Moisture Calibration.
- Hold the sensor in the air (or dry soil). Under Dry Calibration press Start to see the live raw number. When it stops changing, press Save.
- Put the sensor in very wet soil (or water, only up to the line). Under Wet Calibration press Start, let the number settle, then press Save. The bottom line shows Raw, Dry and Wet.
- Test dry, damp and wet soil on the dashboard. If the percentage moves the wrong way, redo both ends and check the sensor connection.
Why Soil Moisture Matters
Too much water is wasteful and can rot roots. Too little stresses plants and slows growth.
Real-World Applications
Soil sensors run automatic watering on farms and in smart gardens. NASA's SMAP satellite even maps soil moisture worldwide every few days!
For Teachers
Students may think the sensor "detects water molecules", but in fact its coated blade holds more charge when water is near (capacitance). No current flows through the soil. Bring real plants to test if you can.
For Parents
Your child is learning how sensors help farmers save water. Soon they can build systems that water plants automatically.
Try It: Calibrate the Soil Sensor
Interactive demo · no kit needed
Play with a pretend MakerBuddy board and its web page, right here.
Follow the steps one at a time. Got the kit? You can switch to the real board inside.
Hands-On Activity: Soil Moisture Testing
Activity Duration: 20-25 minutes
Connect, calibrate and test the sensor dry, moist and wet.
Step-by-Step Instructions:
- Identify the Sensor: Find the soil moisture sensor, a flat, coated blade with a line printed near the top. Only the part below the line may go into soil or water; the rest must stay dry.
- Locate the Connector: First unplug the USB cable, then plug the sensor's cable into the connector marked 32 (bottom of the right-hand edge). The plug fits only one way, so never force it. Plug the USB cable back in.
- Open the Dashboard: Open your dashboard and find the Soil Moisture card (usually 0-100%).
- Air Test (Dry calibration): Hold the sensor in the air, touching nothing. In Soil Moisture Calibration, under Dry Calibration press Start, let the number settle, then press Save. The card now reads 0%, which means 'as dry as my Dry setting'.
- Wet Test (Wet calibration): Dip the sensor into a cup of water only up to the line, never deeper. Under Wet Calibration press Start, let the number settle, then press Save. The card now reads 100%.
- Moist Soil Test: Wipe the sensor dry. Mix a little water into the soil and push the sensor in up to the line. Watch the percentage rise as water reaches the soil around the blade, and record it.
- Recovery Test: Take the sensor out of the soil, wipe it dry with a paper towel and watch the percentage drop back.
What Students Should Observe:
- Dry gives a low percentage, wet a high one
- The percentage rises as moisture increases
- In water the card catches up within about 2 seconds (it gets a new reading about every 2 seconds)
- The percentage drops when the sensor is dried
- Steady readings need the blade pushed into the soil up to the line, with soil pressed gently around it
Important Note
Never put the sensor in water above the line. Water will damage the electronics at the top. Wipe the sensor dry after each use, because water soaking into the blade's edges can damage it over time. Keep the cable plug and the MakerBuddy board dry.
Challenge
Your Mission: The Thirsty Plant Detective
Be a plant scientist and build a plant-monitoring system!
Challenge 1: The Thirsty Plant Test
Use a potted plant (or a cup of soil):
- Push the sensor into the soil and record the first reading
- Water the plant and watch the percentage rise live
- Leave it in and check every 5 minutes for 20 minutes
- Make a data table: Time vs Moisture Level
- Notice how fast the moisture falls as the soil drains
Challenge 2: Moisture Mapping
Test different materials and make a "moisture map":
- Test dry sand, wet sand, dry soil, wet soil, gravel, clay
- Record each reading
- Rank them from driest to wettest
- Which holds water best? Which drains fastest?
- How does this relate to real farming? (Clay soil vs sandy soil)
Challenge 3: The Watering Threshold
Find your plant's watering threshold:
- Research how damp most houseplants like their soil. MakerBuddy's % sits between your own Dry and Wet settings, so use your Challenge 1 readings to choose your own threshold (many classes start around 40%).
- Set a "watering threshold" (e.g., below 40% means the plant needs water)
- Note when your plant reaches it
- In Term 4, we'll automate watering using this threshold!
Discussion Questions:
- Why does wet soil change the sensor's reading? (Water stores charge much better than air or dry soil, so the coated blade holds more charge in wet soil.)
- How could farmers use thousands of these sensors? (Watering only where needed, saving water)
- What happens if you water plants too much? (Root rot, roots short of oxygen, nutrients washed away)
- How does this sensor help save water? (Water only when needed, not on a fixed schedule)
Bonus Challenge: Draw a "smart plant pot": soil moisture sensor → MakerBuddy → water pump (actuator). When moisture is low, it waters the plant. What threshold would you use? How often should it check?
Sustainability Connection
Farms use a large share of the world's fresh water. Smart watering with soil moisture sensors can save water, because plants get water only when the soil is dry.
Key Takeaways
✓ Capacitive Sensing
The soil sensor notices water by how much charge its coated blade holds. MakerBuddy turns the raw number into 0–100% using your Dry and Wet calibration. The same idea is used in touch screens.
✓ Agricultural IoT
Soil sensors let farms water only when and where needed, saving water and money.
✓ Moisture Measurement
Each plant and soil type has its own best moisture range.
✓ Automation Foundation
In Term 4 you'll write rules like "IF Soil Moisture < 40 THEN switch the relay on" (in the kit, the relay stands in for a water pump): sense → decide → act.
Coming Up Next Week:
Week 19: Visual Output (LCD Display)
Learning Support
Use this quick guide to help every learner understand, practice and explain Smart Agriculture (Soil Moisture).
For students
Try it in three steps
- Say it: explain the main idea in one simple sentence.
- Show it: Calibrate the soil sensor (Dry in air, Wet in water), then read it in dry, damp and wet soil.
- Stretch it: Determine critical threshold where a plant needs immediate watering.
Checkpoint: What was your input, what decision or process happened, and what was the output?
For teachers & parents
Coach without taking over
- Before: ask the learner to predict what will happen and why.
- During: use prompts such as “What changed?” and “What could you test next?” instead of giving the answer.
- After: ask for a 30-second explanation, one piece of evidence and one improvement.
Look for: safe handling, correct vocabulary, a result the learner can demonstrate, and an explanation in their own words.
Did you get it?
Three quick questions. Pick an answer to see whether it is right, and why.
1How does the kit's capacitive soil sensor notice water?
2Why do you calibrate the Dry and Wet endpoints yourself?
3How far into water may you put the soil sensor?