What it shows
A motor spinning an unbalanced weight shakes the whole brush. The angled bristles push forward on every shake and slip backward on the return, so the vibration turns into movement in one direction.
The science
When the motor shaft carries a weight on one side only, the centre of mass of the shaft whirls round in a circle and the motor body is thrown the opposite way on every turn, hundreds of times a second. The bristles of the brush are slightly angled. On each shake the brush hops up a little and the slanted bristles land and push the body in the direction they lean. This is the same principle that moves vibrating phones across a table, used on purpose.
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 cheap plastic scrubbing brush or a large toothbrush head
- A 3 V DC toy motor
- A blob of hot glue or a small bead and tape for the off-centre weight
- A 2 x AA battery holder with a switch and cells (3 V)
- Double-sided tape or hot glue
- Two pieces of wire
Safety
For this project: An adult must handle the hot glue gun. Keep fingers away from the spinning shaft.
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.
Press a small bead or blob of hot glue onto one side of the motor shaft so the shaft is clearly unbalanced.
Spin the shaft by hand to check the weight is firmly fixed and does not touch the motor body.
Stick the motor onto the back of the brush with hot glue or double-sided tape, shaft pointing sideways off the edge.
Stick the battery holder next to it, keeping the weight roughly in the middle of the brush.
Connect the motor wires to the battery holder wires; the motor has no polarity to worry about for shaking.
Switch on and put the brush bristles down on a smooth table; it should buzz and move.
If it only spins in circles, move the battery holder to balance the brush, or trim bristles on one side.
Mark a start line and time it over 50 cm; try a heavier weight and compare.
What you should see
The bristlebot scuttles across the table in a steady direction and covers 50 cm in a few seconds.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why must the weight be off centre?
A balanced shaft would spin smoothly and produce no shaking at all.
2Why does it move forward and not just shake on the spot?
The angled bristles grip on one half of each shake and slip on the other.
3What happens if you reverse the motor wires?
The shaft spins the other way; the direction of travel depends mostly on the bristle angle.
4Where is an unbalanced motor used on purpose?
In the vibration alert of a phone or a game controller.
Take it further
Build a second bristlebot and race them, changing the weight and bristle angle to see which design wins.
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
Motors, Gears and Motor Drivers
Make things move: DC motors, servos, steppers and how to drive them