What it shows
Damp soil conducts electricity far better than dry soil. A transistor circuit lights a red LED when the soil resistance between two nails rises, which tells you the plant needs water.
The science
The 220 kΩ resistor feeds about 38 µA to the base. If the soil is dry its resistance is huge, so all this current goes into the base, the transistor conducts and the LED lights. When the soil is damp, its resistance between the nails drops to some tens of kilohms and diverts the base current to ground, keeping the base below 0.7 V and the LED off. Water is not a good conductor by itself, but dissolved salts in soil make it conduct.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two stainless steel nails or screws about 5 cm long
- One BC547 transistor
- A 220 kΩ resistor and a 470 Ω resistor
- A red LED
- A 9 V battery with snap connector
- A breadboard, wires, a potted plant and a cup of water
Safety
For this project: Battery power only, with the breadboard well away from the pot and the water. Do not leave the nails connected for days: they corrode, so switch off between checks.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Push the two nails into the soil about 3 cm apart and 4 cm deep.
Place the BC547 on the breadboard and connect its emitter to the negative rail.
Connect the 220 kΩ resistor from the 9 V rail to the base.
Connect one nail to the base and the other nail to the negative rail.
Connect the 470 Ω resistor from the 9 V rail to the LED long leg, and the LED short leg to the collector.
Snap on the battery with dry soil: the LED should be lit.
Slowly add a little water and note when the LED goes off.
If it switches too early or too late, use a 100 kΩ or a 470 kΩ resistor in place of the 220 kΩ and test again.
What you should see
The LED is lit when the soil is dry and goes out soon after the soil is watered, showing the soil resistance change.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does damp soil conduct better?
Dissolved salts give ions that carry current through the water.
2What happens to the base current when the soil is wet?
It is diverted to ground through the soil, so the transistor turns off.
3Why do the nails corrode?
Current in a damp, salty medium causes electrolysis at the metal.
4How can you change the dryness level that triggers the LED?
Change the 220 kΩ resistor value.
Take it further
Compare the LED switching moment for sandy soil, garden soil and clay and explain the differences.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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