What it shows
Raindrops bridge the gap between two sets of foil strips that are close but not touching. The water lets a tiny current flow, which a Darlington pair of BC547 transistors amplifies to sound a buzzer, warning you to bring the washing in.
The science
The sensor is two combs of foil strips laid between each other like interlocking fingers with a 3 mm gap. Dry, the gap has an almost infinite resistance. A raindrop lying across any two neighbouring strips has a resistance of a few hundred kilohms, enough to let a few microamperes into the base of the first transistor. The Darlington pair multiplies that by about 40,000, turning on the buzzer in the collector circuit. The 100 kΩ pull-down resistor keeps the base at ground when the card is dry.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A plastic sheet or laminated card about 10 x 15 cm
- Aluminium foil cut into 5 mm strips
- Two BC547 transistors
- A 100 kΩ resistor and a 10 kΩ resistor
- An active buzzer rated 3 to 12 V
- A 9 V battery with a snap connector
- A breadboard, jumper wires, glue and a dropper of water
Safety
For this project: Battery power only. Keep the breadboard and battery indoors and dry; only the sensor card goes outside.
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
9 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.
Glue five foil strips onto the plastic sheet in parallel lines 6 mm apart, joined along one edge by a foil bar to make the first comb.
Glue four more strips between them, joined along the opposite edge, so the two combs interlock with a 3 mm gap everywhere.
Check with a multimeter or by eye that no strip of one comb touches the other comb.
Tape a wire onto each comb bar, and lead both wires to the breadboard.
Build the Darlington pair: first emitter to second base, both collectors joined to the buzzer negative lead, buzzer positive to 9 V.
Connect the second emitter to ground, the 100 kΩ resistor from the first base to ground, and the 10 kΩ resistor from the first base to one comb wire.
Connect the other comb wire to the 9 V rail and snap on the battery; the buzzer should stay silent.
Drip water onto the card with the dropper: the buzzer sounds. Wipe it dry and it stops.
Mount the card tilted outdoors under the open sky so drops land on it but run off after rain stops.
What you should see
The buzzer sounds within a second of the first drops landing on the foil combs and goes quiet once the card dries.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why are the strips interleaved rather than just two plates?
Interleaving gives many gaps close together, so even a single drop is likely to bridge one.
2Why does rainwater conduct?
It contains dissolved gases and dust salts that form ions which carry charge.
3What stops the alarm sounding on its own in humid weather?
The 100 kΩ pull-down holds the base near zero unless real water bridges the strips.
4Why tilt the sensor?
So water drains off after the rain and the alarm resets by itself.
Take it further
Replace the buzzer with a 5 V relay module and let the relay switch on a bigger battery-powered lamp indoors.
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.