What it shows
A magnet on a spoke closes a reed switch on the fork once every turn, flashing an LED. Counting flashes in a known time and knowing the wheel circumference gives the bicycle speed, the same method a bike speedometer uses.
The science
The LED, the 150 Ω resistor, the reed switch and the battery form a simple series loop that is open except when the magnet passes. Each wheel revolution brings the spoke magnet past the reed switch, closing it for a fraction of a second and flashing the LED. The distance covered per flash is the wheel circumference, so speed equals circumference times flashes per second. A 26 inch wheel has a circumference of about 2.07 m, so 5 flashes a second is roughly 10 m per second or 37 km per hour.
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 normally open reed switch
- A small magnet and cable ties or tape
- One red LED and a 150 Ω resistor
- A 2 x AA battery holder with cells (3 V)
- About 1 m of twin wire
- A bicycle, a measuring tape and a stopwatch
Safety
For this project: Test with the bicycle upside down or on a stand, never while riding on the road. Keep hands clear of the spokes.
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.
Tape the magnet to a spoke near the rim, and tape the reed switch to the fork so the magnet passes within 1 cm of it.
Connect the battery red lead to the 150 Ω resistor and the resistor to the LED long leg.
Connect the LED short leg to one reed switch lead and the other reed switch lead to the battery black lead.
Tape the LED and battery to the handlebar with the wire running down the fork.
Turn the bicycle upside down and spin the wheel slowly by hand: the LED flashes once per turn.
Measure the wheel circumference by rolling it one full turn along the floor and measuring the distance.
Spin the wheel, count the flashes in ten seconds and calculate the speed in metres per second.
Convert to kilometres per hour by multiplying by 3.6 and record several trials.
What you should see
The LED flashes exactly once per wheel turn, and the flash count gives a speed that agrees with timing one revolution by hand.
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 flash only briefly?
The reed switch is closed only while the magnet is passing within a centimetre of it.
2How do you turn flashes into speed?
Multiply flashes per second by the wheel circumference.
3Why mount the magnet near the rim rather than the hub?
Near the rim it passes the reed switch quickly and clearly, giving a clean pulse.
4What would a bigger wheel do to the flash rate at the same speed?
Fewer flashes per second, because each turn covers more distance.
Take it further
Feed the reed switch pulses into an Arduino and display the speed on a small screen.
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
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it