What it shows
Each question fastener is secretly wired to its correct answer fastener behind the board. Touching the two probes to a matching pair completes the circuit and lights the LED, so the board tests both knowledge and the idea of a closed loop.
The science
The LED, resistor and battery are joined in series with the two probe wires, leaving a gap that only the board can close. Behind the board every question pin is connected by foil or wire to the pin of its correct answer, and the links cross each other without touching because tape insulates them. When the probes touch a correct pair the hidden link closes the gap and current flows; a wrong pair has no link, so the LED stays dark. The 150 Ω resistor limits the LED current to about 8 mA from the 3 V supply.
Materials
Cost: under Rs 100. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A sheet of stiff cardboard about A3 size
- Twelve brass paper fasteners (split pins)
- Aluminium foil strips or thin insulated wire for the hidden links
- One red LED and a 150 Ω resistor
- A 2 x AA battery holder with cells (3 V)
- Two 40 cm probe wires with bare tips
- Tape, a marker and a list of six questions with answers
Safety
For this project: Split pins have sharp points at the back; an adult should press them through and fold them flat.
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.
Write six questions down the left of the board and the six answers in a jumbled order down the right.
Push a brass fastener through the card beside every question and every answer, and fold the legs flat at the back.
At the back, run a foil strip or wire from each question fastener leg to its correct answer fastener leg, taping it to both legs.
Cover each link with tape before laying the next one across it so crossing links never touch.
Wire the battery red lead to the 150 Ω resistor, the resistor to the LED long leg, and the LED short leg to probe one.
Wire probe two straight to the battery black lead; the probes are now the two ends of the open circuit.
Touch one probe to a question pin and the other to the right answer: the LED lights. Touch a wrong answer: nothing.
If a wrong answer also lights, two links are touching at the back; find and re-tape them.
Decorate the front and challenge the class.
What you should see
The LED lights only for correct question and answer pairs, and stays dark for every wrong pair.
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 a wrong answer not light the LED?
There is no hidden link between those two pins, so the circuit stays open.
2What stops crossing links at the back from short-circuiting?
A layer of tape insulates each link from the ones crossing it.
3What would happen without the 150 Ω resistor?
The LED would draw too much current and could burn out.
4Is brass a conductor or an insulator?
A conductor; it is an alloy of copper and zinc.
Take it further
Add a second LED of another colour and a buzzer so the board can give sound and light for a correct answer.
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
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each
Science projects
Best Out of Waste Science Projects
Working models from bottles, cardboard, CDs and old parts
Find out more about electronics and robotics
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