What it shows
A circuit is only complete when a wire joins the right pair of terminals. Touching a probe wire to the correct answer closes the loop and lights an LED, while a wrong answer leaves it open.
The science
Current flows only around a complete conducting loop. Each question has a split pin on the left and each answer a split pin on the right, with a hidden wire at the back joining the right pairs, not the neighbouring ones. The probe from the battery touches the question pin and the other probe touches an answer pin: the loop through the LED completes only when a wire joins those two pins.
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.
- A sheet of stiff card about 30 cm x 20 cm
- Ten brass paper fasteners (split pins)
- Insulated wire in five lengths
- Two probe wires with bare ends
- A 2 x AA battery holder with two cells
- One LED and a 150 Ω resistor, or a 3 V buzzer
- Tape and a marker
Safety
For this project: Cells only. Cover the back wire joints with tape so they cannot touch each other by accident.
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.
Write five questions down the left side and five answers in mixed order down the right side.
Push one split pin through the card next to each question and answer, and fold the legs flat at the back.
At the back, join each question pin to its correct answer pin with a wire, wrapping the bare ends round the pin legs.
Tape all wires flat and keep wires from touching each other where they cross.
Connect the battery holder red wire to the LED through the 150 Ω resistor, and the LED short leg to probe wire one.
Connect the battery holder black wire to probe wire two.
Touch probe one to a question pin and probe two to an answer pin. The LED lights only for the right pair.
Swap the answers to new positions and rewire the back to make a second quiz.
What you should see
The LED lights for correct matches and stays off for wrong ones, and the quiz can be reused by changing the hidden wires.
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 the LED light only for the correct pair?
Only a correct pair has a wire joining it, so only then is the loop complete.
2What does the 150 Ω resistor do?
It protects the LED by limiting the current to a safe value.
3What would happen if two back wires touched?
A wrong pair could connect and light the LED, giving a false answer.
4Is the board using direct or alternating current?
Direct current from the cells.
Take it further
Add a buzzer for wrong answers using a second probe pair and a second circuit.
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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