Bicycle dynamo lamp
A cheap bike dynamo lights an LED. A great way to see how a generator works.
The big idea
A dynamo is a very small generator. When it spins, it lights a lamp. It needs no battery.
You will need
Tick things off as you gather them.
Step by step
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Step 1: Look at the dynamo
Find the roller at the top and the wire terminal. The roller is the part that touches the tyre.
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Step 2: Fit it to the bike
Mount the dynamo on the fork or frame so its roller touches the side of the tyre. Tighten it, leaving it able to swing in and out.
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Step 3: Connect the lamp
Connect the dynamo to the lamp with the wires. One wire goes to the lamp and the other earths to the frame.
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Step 4: Turn the wheel by hand
Lift the wheel and spin it slowly with your hand. Watch the lamp flicker, then glow steadily.
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Step 5: Spin it faster
Spin it faster. Does the lamp get brighter? What happens when you stop the wheel?
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Step 6: Measure the voltage
If you have a multimeter, measure across the lamp at different speeds. Plot speed against voltage.
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Step 7: Open it up
With a teacher, open an old dynamo and look inside. Find the magnet and the coil of wire.
What to look for
- The lamp lights only while the wheel turns.
- A faster wheel makes a brighter light.
- A dynamo has no battery, yet it still makes power.
Why it works
Inside the dynamo is a magnet that spins next to a coil of wire. A moving magnet pushes electrons along the wire in the coil, so a current flows. The faster the magnet spins, the stronger the push. Stop spinning, and the current stops. Every power station in the world generates electricity this way, with a bigger magnet and a bigger coil.
Try this next
- Fit a capacitor so the lamp keeps glowing for a moment after the wheel stops.
- Compare a dynamo with the motor generator in your wind turbine.
- Work out how fast you need to ride to light the lamp fully.
Stay safe
Check the wiring is clear of the spokes. Do the first tests with the bike on a stand.
Link to the grid
Generators in power stations are huge dynamos. The idea does not change: a magnet and a coil, turned by something moving.
Learn how the grid worksMore wind, water and muscle experiments
Paper or plastic bottle wind turbine
Blades cut from a bottle spin a small DC motor that works as a generator and lights an LED.
Water wheel generator
Spoons or bottle caps on a wheel catch falling water and turn a motor to make electricity.
Pedal-powered generator
An old bicycle turns a DC motor to light bulbs and shows how much effort electricity takes.