The science idea
A bristlebot is one of the simplest robots in the world: it has a body, a motor, a power source and legs, and it moves by itself. It does not have wheels. Its legs are the bristles of a toothbrush, and its motor does not turn wheels at all, but just shakes. Even so, it walks. Building one teaches you about circuits, vibration and friction at the same time.
The motor is the source of the vibration. On its shaft you fix a small weight on one side, so that the shaft is out of balance. When the shaft spins fast, the weight goes round in a circle, and because it moves in a circle it needs a force towards the centre, called the centripetal force, which is provided by the shaft. In return the weight pulls the shaft and the motor towards itself. As the weight goes round, the direction of this pull keeps changing, so the motor, and anything fixed to it, is shaken about. This is the same trick used in a mobile phone’s vibration alert.
The shaking alone would only make the bot jitter on the spot. The bristles turn it into walking. The bristles lean in one direction, like the teeth of a comb. When the body is pushed down by the vibration, the bristle bends, and because it leans, its base moves forward. When the body lifts, the bristle springs back, but the tip swings forward through the air or slides, because it is no longer pressed to the ground. After many cycles, each moving the body forward by a tiny distance, the bot goes forward at a few centimetres per second. The bot walks in the direction in which the tips of the bristles trail behind.
The electrical side is a simple series circuit. The cell is 1.5 V, and the motor is joined to its two ends through a switch (or just a lead you touch to the cell). Current flows out of the positive end of the cell, through the motor, and back to the negative end. The current makes a magnetic field in the coil of the motor, which pushes on the magnets and turns the shaft. Reversing the cell makes the motor spin the other way but the bot still vibrates.
This is an ideal project for experiments: you can change the size of the weight, the position of the weight, the number of bristles, the angle of the bristles or the cell, and measure the speed over a fixed distance. Each change is a variable. Keep the others the same, repeat each race three times, and draw a graph.
Materials and tools
Cost: Free
An old toothbrush, a small motor from a broken toy or pager, and a used AAA cell are enough. If you must buy a small 1.5 to 3 V motor it costs about Rs 15 to Rs 40.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Old toothbrush (or a scrubbing brush head) | 1 | From the bathroom. Choose one with soft bristles which lean evenly. A flat-cut head is best. |
| Small DC motor, 1.5 V to 3 V (type 130, toy-car motor or pager motor) | 1 | From an old toy, a dead pager, a CD tray or a mobile phone (vibration motor). An adult can take it out. |
| AAA or AA cell, 1.5 V | 1 | From a remote control that still works. Not a button cell. |
| Piece of rubber eraser, a small lump of clay or a bit of cork for the off-centre weight | 1 piece about 1 cm across | Cut from the end of a pencil eraser. |
| Double-sided tape or a little hot glue | 1 small piece | To fix the motor. Ask an adult for hot glue. |
| Thin insulated wire with bare ends (or the motor’s own wires) | 2 pieces, each 6 cm | From old wire or earphones. |
| Cellophane tape or electrical tape | 1 roll | To hold the cell and the wires. |
| Googly eyes, marker pens (optional) | A few | For decoration. |
Tools
- Scissors or cutting pliers to cut the toothbrush (adult)
- Small screwdriver
- Ruler and a stopwatch
- A flat smooth table for testing
- Pencil
Safety first
Read this before you start
- An adult should cut the toothbrush and, if needed, use hot glue. Cut plastic can be sharp, and hot glue can burn.
- Use only an AAA or AA cell. Do not use a coin or button cell, which can be dangerous if swallowed, and do not use any larger battery or any power supply.
- If the motor is blocked and cannot turn (for example, the weight is stuck), it takes a large current and gets hot. Disconnect the cell at once.
- Do not use the cell’s ends to short the wires together. Keep the wire ends apart, and take the cell out when the bot is not in use.
- Do not use a toothbrush that you or someone else still uses. Wash your hands afterwards. Keep small parts away from younger children.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Prepare the toothbrush head
An adult cuts the head off the toothbrush about 1 cm behind the bristles, leaving a head about 5 to 6 cm long. Cut it so that the cut end is flat. Check that the bristles slope one way.
Tip: Look at the head from the side. Bristles that lean (slope) evenly make the bot walk straight.
Test the bristles
Place the head on the table and push it gently forward and backward with your finger. It should slide more easily in one direction than the other. Mark the direction in which it slides easily.
Common mistake: Skipping this test. You must know which way the bot will go, so that you can fix the motor the right way round.
Check the motor
Touch the motor wires to the ends of the cell (one wire on the + end, one on the − end). The shaft should spin quickly. If it does not, try another motor or a fresh cell.
Tip: Spin the shaft with your finger first. It should turn freely.
Make the off-centre weight
Cut a small piece of eraser about 1 cm × 0.5 cm. Make a small hole in it with a pin, and push it onto the motor shaft so that the weight sticks out to one side. If the shaft is too thin, tape the eraser to it.
Common mistake: Centring the weight. An evenly balanced weight does not shake the motor, so there is no vibration.
Check the clearance
Turn the shaft slowly by hand: the weight must go round without touching the motor or the body. Make sure the weight hangs out clear over the edge.
Tip: Let the shaft stick out over the rear edge of the head, so the weight swings freely.
Fix the motor
Stick the motor on top of the head with double-sided tape or hot glue, near the end that will be the back, with the shaft pointing sideways or backwards. Press firmly.
Common mistake: A loose motor. The vibration is wasted if the motor is not fixed firmly to the body.
Attach the cell
Lay the AAA cell along the top of the head, in front of the motor, with a piece of tape across it. Check that its two ends are exposed for the wires.
Tip: Put the cell so the weight is slightly towards the back. Too much weight at the front makes the bot dig in.
Connect the wires
Tape one motor wire to the + (button) end of the cell and the other to the − (flat) end. Keep the wires apart. The motor should start and the bot will shake.
Common mistake: Letting the two bare wires touch each other. That is a short circuit, and the cell drains and the wires warm up.
Test it
Put the bot on a smooth table. It should creep forward in the direction the bristle tips trail. If it spins on the spot, the weight is too large or on the wrong side. Adjust the position or size of the weight.
Tip: Try the weight on the left and the right of the shaft. It changes how the bot steers.
Steer it
Bend or trim some bristles on one side, or put the weight off centre, to make the bot turn left or right. Draw a curved track with chalk and see if it follows.
Common mistake: Cutting too many bristles at once. Trim a few at a time and test again.
Race and measure
Mark a 30 cm track. Time each run with a stopwatch from the start line to the finish line. Do three runs for each setting (a smaller weight, a bigger weight, trimmed bristles) and write them in the table. Speed = distance ÷ time.
Tip: Use the same smooth table every time, and put the bot in the same starting spot.
Decorate and show
Add googly eyes and draw a face, then hold a bristlebot race with friends or build a small arena with walls from books.
Tip: Take the cell out after each session so the bot does not run down.
How it should look when it is finished
A tiny robot about 6 cm long, made of a toothbrush head with a small silver motor and a lump of eraser on one end, and a thin AAA cell taped on top. When the wires touch the cell, the bot buzzes and starts to trot along the table on its bristle legs, wobbling slightly, with a pair of googly eyes looking ahead.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
The cell pushes a current through the motor. In the motor the current makes the shaft spin quickly. Because the weight on the shaft is not in the centre, the spinning shaft is out of balance and shakes the motor and the toothbrush body many times a second.
The bristles are like tiny leaning legs. When the body is pushed down, the bristle is bent and its base moves forward over the sticking tip. When the body bounces up, the tip is free and swings forward. So at each shake the body moves forward by a very small step, and the steps add up to a steady walk.
Changing the weight changes the force and the frequency of the shaking. Too little and the bot barely vibrates, and too much and it jumps and spins. The best setting is found by testing.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The motor does not spin | A loose wire, a flat cell, or a stuck shaft. | Check the wires touch the metal ends of the cell, try a fresh cell, and turn the shaft by hand. |
| The bot vibrates but does not move forward | The weight is too small, the table is too rough, or the bristles are too stiff. | Use a larger weight, try a smooth surface and use soft bristles. |
| The bot spins round in circles | The weight is off to one side of the bot or the bristles are uneven. | Move the motor to the centre line or trim the bristles on the outside of the turn. |
| It goes backwards | The bristle tips lean the other way from what you assumed. | Turn the bot round or change the way the bristles lean, and test again. |
| The motor slows down or gets hot | The weight touches the body or the cell is weak. | Free the weight and replace the cell. Never leave it on if the motor is hot. |
| The motor falls off | The tape is weak or the surface is dusty. | Use stronger tape or hot glue, and wipe the plastic first. |
Test and record
Use the same track and the same cell for all runs. Do three runs for each setting and find the average. Speed = 30 cm ÷ average time.
| Setting | Run 1 (s) | Run 2 (s) | Run 3 (s) | Average time (s) | Speed (cm/s) |
|---|---|---|---|---|---|
| Small weight (about 0.5 cm) | |||||
| Large weight (about 1 cm) | |||||
| Bristles trimmed shorter | |||||
| Weight at the other side |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- Why does the motor need an off-centre weight?
An unbalanced weight goes round with the shaft and makes the motor shake. A motor with balanced shaft would just spin without shaking the body.
- What provides the centripetal force on the weight?
The shaft. The weight is pulled inward by the shaft so that it moves in a circle, and the weight pulls back on the shaft, which shakes the motor.
- Why do the bristles make the bot walk forward?
They lean, so they bend and spring back asymmetrically. The base moves forward when the bristle is pushed down, and the tip slides forward when it lifts.
- Which way does the bot walk, and how can you tell?
In the direction opposite to the lean of the tips (the tips trail behind). Test it by pushing the head forward and backward and see which way is easier.
- Draw the circuit of the bristlebot.
A 1.5 V cell, a switch (or a touching wire) and a motor in one series loop, current leaving the + end of the cell.
- What happens if the weight is too big?
The bot jumps, spins or loses grip, and the motor may slow down. Too much weight also uses more current.
- What energy changes occur in the bristlebot?
Chemical energy in the cell turns into electrical energy, then into kinetic energy of the motor and the shaking, and finally into the kinetic energy of the bot, plus heat and sound.
- How did you keep your race fair?
By using the same track, the same cell and the same start position, and changing only one thing at a time.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a bristlebot and to investigate how the size of the off-centre weight affects its speed.
- Apparatus
- Toothbrush head, small DC motor, AAA cell, eraser, tape, wires, ruler, stopwatch.
- Procedure
- Describe how you prepared the head, fixed the motor, made and fixed the weight, connected the cell, and measured the time over 30 cm for different weights. Include the diagrams.
- Observation
- The table of times for each setting and the speed calculated from the average.
- Result
- The fastest setting was ... with a speed of ... cm/s. The bot walked in the direction that the bristle tips trail.
- Conclusion
- The off-centre weight makes the motor vibrate, and the leaning bristles convert this shaking into forward motion. The size and the position of the weight affect the speed.
Take it further
- Make a bristlebot with two motors, one on each side, and control the turn by switching one motor on.
- Build a maze or a racing track and hold a competition with friends.
- Add a second cell in series (3 V) only if your motor is rated for 3 V, and see what changes.
- Test different surfaces: glass, wood, cloth and paper, and compare the speeds.
- Try a toy brush, a scrub brush or a hairbrush as a body and compare.
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Free ebooks for this project
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