This document summarizes a student science fair project to build an electronic soil moisture sensor using a simple circuit. The circuit uses probes inserted in soil to measure electrical resistance, which indicates how wet or dry the soil is. If the soil is dry, a LED will light up, and if the soil is wet, the LED will stay off. The students hypothesized this circuit could help determine if plants need watering. They assembled the circuit using common electronic components, tested it, and experimented with monitoring plant moisture levels and watering based on the sensor readings. The sensor worked but had limitations detecting moderate moisture levels. The students learned about simple circuits and using resistance measurements to gauge soil moisture.

1. How to Know How Much Water in Soil?
Science Fair
NATHANAEL, ANNA, SHEILA

2. Have you ever got trouble with…

3. Watering Your Plants?

4. May be
too much Water?

5. Or
too little water?

6. Is there anything that can save us?

8. What we are going to make
Electronic Soil
Moisture Sensor

9. Hypothesis & Variables

10. Variables
Independent variables
the amount of water
Dependent variables:
the electronic resistance of the soil, the electrical conductivity
Control variables
The surface area of the probes, the distance between the probes
🚿
💡
🔌

11. Hypothesis
For the end result of the experiment：
If the soil is too dry, then the LED will turn on.
If the soil is wet, then the LED will stay off until the soil dries out
again.

12. Materials

13. For our experiment, we will need to buy...
Breadboard
9 V battery
9 V battery snap
connector
CD4011 NAND
gate integrated
circuit (IC)
100 kΩ resistors
(2) (brown, black,
yellow, gold
stripes)
470 Ω resistor
(yellow, purple,
brown, gold
stripes)
Red LED
10 MΩ resistor
Jumper wires
(assorted)
Alligator clip
leads (2)
Soil
Popsicle sticks
Aluminum foil
Small block of
stiff foam or
wood
Small plastic
container with lid
Power drill or
hobby knife
Safety goggles
• Hot glue or
waterproof •
silicone sealant
We can buy it at/on…
www.taobao.com
www.jd.com
www.amazon.com
www.ebay.com
Approximate cost 120 CNY

14. Process

15. Part 1
Prepare the materials and dig the
right amount of soil

16. Part 2
Start to Assembling
the Circuit

17. Part 2
•4011 NAND gate, Rows1-7, straddling the
middle of the bread board, with semicircular
notch facing up
•10 MΩ resistor, B2 to (-) bus
•470 Ω resistor, B3 to B11
•Red LED, Long lead to A11, short lead to (-)
bus
•100 kΩ resistor, C2, other lead free
•100 kΩ resistor, D1, other lead free
•9V battery and snap connector, Red lead to (+)
bus black lead to (-) bus
•Jumper wires (4), B1 to (+) bus, J1 to (+) bus,
A7 to (-) bus, Left (+) bus to right (+) bus

18. Part 3
Test the Circuit

19. Part 4
Experiment

20. Group #1(use the Electronic Soil
Moisture Sensor): Always observe the
state of the LED, according to the
prompt and then water.
Group #2(do not use the
Electronic Soil Moisture Sensor):
Keep a
fi
xed amount of water
poured every day.

21. Result

22. 💡
+ =
💡
💡
+ =
💡
+ 💧

23. BRIGHTNESS
NUMBER
💡 💧

24. 💡
💧
💧
💧

25. 💡
💧
💧
💧
When then
💧
💧
💧

26. 💡
💧
💧
💧
When then
💧
💧
💧💧
💧
💧

27. Experiment
order
U/V R/Ω
The brightness of
the bulb
U/R=I
(1) 2.5 2.8 Normal light 0.89
(2) 2.5 4.5 Darker 0.55
(3) 2.5 5.8 Dark 0.43
(4) 2.5 6.7 Slightly bright 0.37
(5) 2.5 7.8 Not bright 0.32
This is the result of our resistors measurement
using different resistance values

28. Discussion

29. Circuit diagram
+
—
Formula
I=U/R
Our experiment proved
that our original
hypothesis was correct.

30. Circuit diagram
+
—
Our principle is to
make a simple bulb
circuit, and this circuit
is parallel circuit …

31. resistor
Why we use this
things…

32. What knowledge can
we use during the
experiment?
Formula
I=U/R

33. Conclusion

34. NOOOO! YES!

35. Some Problem…

36. 🥀
In order to save the cost, we use a very simple device.
Because the device is very simple, so we used to measure the dry
degree of soil resistance is very small, lead to soil can only lights in
very dry
But for many plants, let them survive such extreme environment,
they will die soon. Such as rose.
if excessive water will dry death The resistors should normally and
accurately detect whether the soil is wet or dry
💸
💦

38. Don’t at here, if u look at here, you have already lose 5 seconds, haha!
Thank You!