What it shows
A DC motor works backwards as a generator. When its shaft is turned by a spinning tyre it makes a voltage, and reversing the spin reverses the voltage, which lights the other LED of an opposite pair.
The science
Turning the motor shaft makes the coil wires cut the field of its magnets, which induces an emf proportional to the speed. The motor gives a DC voltage whose sign depends on the direction of rotation. Two LEDs joined in opposite directions mean only the correctly pointing LED conducts for each direction. The 100 Ω resistors protect the LEDs when the voltage is high.
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 6 V or 12 V hobby DC motor (the kind in a toy car)
- A rubber roller cut from a cycle tube or a thread bobbin on the shaft
- A bicycle on a stand, or a bicycle wheel on a fork held in a vice
- Two red LEDs and two 100 Ω resistors
- A strip of wood and cable ties to hold the motor
- Two wires and tape
Safety
For this project: An adult must help to hold or stand the bicycle. Keep fingers, hair and clothes clear of the spinning wheel. Do not use a mains-powered driver.
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.
Cable-tie the motor to the wooden strip so the roller on its shaft can press gently on the side of the tyre.
Fix the strip to the frame so the roller touches the tyre with enough pressure to turn without slipping.
Connect each LED in series with a 100 Ω resistor to make two LED branches.
Join the two branches across the motor terminals in opposite directions: the long leg of one to the same terminal as the short leg of the other.
Turn the wheel by hand forward quickly and see which LED lights.
Turn it backward and see the other LED light.
Spin the wheel slowly and quickly and note the change in brightness.
If neither LED lights, spin faster, use a larger roller, or use a 12 V motor with higher output.
What you should see
One LED lights for forward spin and the other for backward spin, and both get brighter as the wheel spins faster.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1How can a motor act as a generator?
Turning its coil in the magnetic field induces an emf by electromagnetic induction.
2Why does the other LED light on reversing?
The induced voltage changes sign, so current flows through the other branch.
3Why does the brightness change with speed?
Faster rotation cuts the field lines faster, producing a larger voltage.
4What is the energy change?
Kinetic energy of the wheel changes into electrical and then light energy.
Take it further
Put a 4 x 1N4007 bridge and a 470 µF capacitor between the motor and one LED to get steady DC light.
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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Twenty classic models, how each works and how to make it