Bristle Brawler

Difficulty

Beginner

Kit Cost

$15

Build Time

1 Weekend

Competition

Sumo · Sprint · Freestyle

The Build

The Bristle Brawler is your entry ticket to robot combat. No programming. No microcontroller. Just pure physics — a salvaged vibration motor, angled bristles, and the ruthless laws of table-top warfare.

Harvest a vibration motor from an old electric toothbrush, mount it on a lightweight chassis with bristles angled at 15 degrees, and watch it scuttle across the arena like a mechanical cockroach. The vibration provides forward momentum; the bristles determine direction and grip. Add a coin cell battery and a tilt switch for auto-start, and you're ready for the sumo ring.

Find an old electric toothbrush — the cheap kind with a single vibration motor. Pop the case open with a flathead screwdriver. Inside you'll find a small DC motor with an offset weight on the shaft. That's your engine.

Desolder or snip the motor leads, keeping as much wire as possible. Test it with a coin cell — it should spin and vibrate. If it's dead, try another toothbrush. Dollar stores are gold mines.

The kit includes pre-cut cardboard chassis templates — two-layer construction with a motor well and bristle mounting slots. If you have a 3D printer, the STL files are included for a more durable frame.

Cardboard build: cut out the base and top plate, fold the tabs, glue them together with the motor sandwiched in the middle. The motor should sit centered, with the weighted side pointing down.

// Chassis dimensions (kit template)
Base: 6cm x 6cm, 1mm cardboard
Motor well: 12mm dia, centered
// ... full template and assembly guide in project files
  

The kit includes pre-cut bristle strips with adhesive backing. Attach them to the underside of the chassis at a 15-degree forward angle. The angle is critical — too steep and the bot flips, too shallow and it just vibrates in place.

Bristle placement determines direction. If the bot spins instead of moving forward, adjust the bristle angles until they're symmetrical. Small tweaks make a big difference.

Connect the motor to the coin cell holder. Add the tilt switch in series — when the bot is tilted to the "on" position, the circuit closes and the motor spins. No solder required if you use the kit's crimp connectors.

Mount the battery holder on the top plate with double-sided tape. The tilt switch goes near the front, secured with hot glue. Test the circuit before final assembly.

// Wiring (no code, just connections)
Coin cell + → Tilt switch → Motor +
Motor - → Coin cell -
// ... full wiring diagram in project files
  

Weight distribution is everything. The kit includes small metal slugs that you can tape to the chassis to adjust balance. If the bot veers left, add weight to the right side. If it flips forward, move weight toward the back.

Test on a smooth surface first — wood or laminate flooring works best. Once it's moving straight, try the arena. Bristle grip changes with surface texture, so you may need to re-tune for competition.

Standard sumo rules: 24-inch diameter ring, tape or chalk boundary, no rails. Bots start facing each other, 6 inches apart. On the signal, tilt your bot to activate the tilt switch. First bot to push the opponent off the ring wins.

Weight limit: 100 grams. No projectiles, no liquids, no damage weapons. Bristles and body contact only. Best of three rounds.