Parts and prices

What it costs

Most of these parts come in packs of 5, 10 or 100, so the price per robot depends on how many robots you build.

• A class or a group, packs split across three or more robots: about $87 per robot from Amazon.
• Same, with the servos and boards from AliExpress: about $55 per robot. Allow 2 to 4 weeks for shipping.
• One robot, every Amazon pack bought once: about $235. You end up with most of a second robot's small parts left over, so find a friend to build one too.
• Name-brand US vendors only (Adafruit, Pololu, DigiKey): about $170 per robot. Two Pololu 5 V 5 A regulators alone are $66. Use these as the fallback when one part from a cheap pack turns out bad.

Prices change. Check each listing before you order.

The one expensive component

The 2S LiPo kit, two batteries and a charger, at about $17. A rolling wheel cannot trail a power cable, so the battery has to ride on the robot, and eight servos need a pack that can deliver 5 A.

Why MG90S and not SG90

The plastic-gear SG90 is about $1 cheaper per servo. In the closed wheel the servos push against each other continuously, and plastic gears strip. The whole robot costs less than two smart servos, so there is room for metal gears.

Tools and consumables

Not counted in the per-robot price:

• FDM printer with a 180 mm bed, or a print service (about $20 to $30 for the set)
• Soldering iron and solder, flush cutters, small Phillips screwdriver, 2.5 mm hex key, calipers
• Multimeter, to check both 5 V rails before anything is plugged in
• Kitchen scale, to measure the magnet pull-off force
• Two USB-C data cables
• LiPo-safe bag or a metal tray for charging
• Superglue, hot glue, double-sided foam tape

Sourcing notes

The table above links one listing per part plus a cheaper or more trustworthy alternative. Listings come and go, so here is what to look for when the linked one is gone or looks wrong. Prices change; check before you order.

Servos: MG90S, 180°

• The listing must say 180°. A 360° or "continuous rotation" MG90S looks identical and will not hold a position. Skip listings that say "90-180°" without saying which, such as B0CH8BFXWW.
• Metal gears, about 2.0 kg·cm at 6 V. Buy 10 for one robot: two are spares, and servos wear out on this robot.
• The wiring plan assumes a 240 mm lead. Check the lead length in the listing. A shorter lead means longer extensions for the servos near the tail.
• Cheaper: Waveshare sells them at $2.59 each, $2.15 each at 10 to 49, shipped from China.
• More trustworthy: DFRobot's DMS-MG90 at $5.90 each ($5.20 at 10+), 180° ±10°, 12 g. Its lead is 180 mm, shorter than the 240 mm the wiring assumes.

ESP32 boards

• Classic ESP32-WROOM-32, not C3, S2 or S3. The classic chip has the 16 PWM channels the firmware uses.
• The listed board is the 38-pin DevKit with USB-C and a CP2102 USB chip. Some similar listings are 30-pin or Micro-USB; check the photos.
• You need two per robot: one rides on the robot, one is the USB dongle. A 2-pack is about $14.
• Alternatives: DIYables' 38-pin USB-C board, about 55 × 28 mm, at $8.99 each. Adafruit's HUZZAH32 Feather ($19.95) is a genuine WROOM-32 but a different shape, so it will not drop into the same layout.

IMUs: GY-521 MPU-6050

• Two per robot. A 5-pack is about $13, which covers two robots with a spare.
• Adafruit's MPU-6050 ($12.95) and RobotShop's ($13.64) are fine sensors but different board layouts from the GY-521.
• Mount both with X toward the head and Z out of the back.

Current sensor: INA219

• Look for a 0.1 Ω shunt (marked "R100"). The common GY-219 board is rated 26 V, ±3.2 A; with a second 0.1 Ω resistor across the shunt it reads to 6.4 A.
• Size matters. The power board fits an INA219 up to 20.5 × 22.9 mm. Adafruit #904 ($9.95, 20.3 × 22.9 mm) fits beside full-size bucks. A 25.6 × 20.5 mm generic board fits only if your Mini560s are at most 26 mm long.
• Elecrow's INA219 module ($15) is a Raspberry Pi Pico-format board and costs more.

5 V regulators: Mini560

• Fixed 5 V only. Mini560 also comes in 3.3, 9 and 12 V versions, so check the "5V" marking. There is no fixed 6 V version, and an adjustable buck is the trap: a mis-set adjustable buck is how a 9 V rail kills eight servos.
• 7 to 20 V in, 5 A out, at most 29 × 18 mm. Expect it to need cooling above about 4 A; stalls must stay short.
• Check the output with a meter before you connect anything.
• A 5-pack is about $12 and covers two robots with one spare. Cheap Mini560s are easiest to find on eBay and AliExpress.
• The trustworthy fallback is Pololu's D24V50F5 at $32.95 each.

Battery and charger

• 2S 7.4 V, 450 to 500 mAh. Look for a kit with two packs and a USB balance charger.
• No thicker than 12.5 mm. The pack stands on its long edge on link s3, and anything thicker eats the clearance for joint 4. Packs sold as 2S 500 mAh range from 47 × 25 × 10 to 56 × 28 × 13 mm, so check. A slightly longer pack is fine.
• The listed URGENEX 450 mAh pack is about 46 × 25 × 9.7 mm and 23 to 25 g, and the kit comes with a 1-to-2 USB charger. The same brand also sells the packs without a charger (B0H7R5R7HS); that is not the one you want.
• Fit XT30 connectors. Small packs ship with PH2.0 or "JST" plugs. PH2.0 is rated about 2 A, and the robot draws 1.8 A continuously while rolling and 5 A in a stall. Put a female XT30 on each battery and a male on the robot. "JST" on a hobby pack usually means the larger RCY/SYP style, not PH2.0; with XT30 fitted it does not matter which you get.
• Alternatives from hobby shops: Tattu 2S 450 mAh 75C ($7.79, 44 × 25 × 14 mm, 29 g, which is thicker than the 12.5 mm limit) and ManiaX 2S 450 mAh 30C ($8.99, 62 × 30 × 11 mm, 31 g), plus an HPI 2S USB balance charger ($11.99). Stock comes and goes.

Magnets

• 6 × 3 mm neodymium discs, two per robot. Many packs sold as 6 × 3 are really 6 × 2.6 mm or thinner; check the listed thickness.
• 1/4 × 1/8 inch discs (6.35 × 3.18 mm) work too, after you change the pocket size in cad/make_parts.py. Apex Magnets sells an adhesive-backed 20-pack (10 N, 10 S) for $9.99; K&J Magnetics' D42 (N42) is about $0.41 each.
• Aim for a pull-off force of 130 to 200 g on a kitchen scale, and set it by shimming the magnet deeper.

O-rings (the treads)

• NBR, exactly 18 mm inside diameter and 2.5 mm cross-section. 18 per robot. The nearest inch size, AS568-017, is too thin.
• They are hard to find on Amazon; some assortment kits include a few. The O-Ring Store sells N2.50X018 (70 durometer) at $0.08 each with an 85-piece minimum ($6.80), enough for four robots. Tameson sells a 100-pack.

Screws

• M3 × 25 mm socket head, carbon or alloy steel (black oxide or zinc plated), because a magnet has to hold them. Not stainless.
• About $8 to $10 per 100. Pololu's M3 × 25 is a Phillips pan head, which is the wrong shape.

Small electronics

• Capacitor: 220 µF 16 V low-ESR. Not 1000 µF: the Mini560 can latch off at start-up into more than about 440 µF.
• Resistors: you need 100 kΩ and 33 kΩ for the battery divider. Check the value table of any kit; the Aniann 1280-piece kit lists both, and smaller kits may not include 33 kΩ. DigiKey sells singles at about $0.10 each, plus shipping.
• Perfboard: double-sided 4 × 6 cm with plated-through holes. Adafruit's 3-pack ($2.50, 14 × 20 holes) does not say whether it is plated through.
• Header pins: 2.54 mm male, one 40-pin strip per robot. DFRobot sells 10 strips for $3.10; Pololu sells single strips at $0.95.
• Servo extensions: 3-pin JR/Futaba. DFRobot sells 150 mm ones at $0.60 each.
• XT30 pairs, 3 A-hold polyfuse (such as RUEF300), 0.1 Ω 1 W resistor, 100 nF ceramic capacitor, M3 ring terminals: any listing that matches the spec in the table.

Wire, ties and filament

• Wire: silicone insulation in 22 AWG (red and black, for battery and servo power), 26 AWG and 28 AWG (signals). Adafruit's 25 ft spool of 26 AWG is $4.95, one colour per spool.
• Zip ties: 2.5 × 100 mm. ServoCity sells 100 for $4.99.
• Filament: PETG, about 150 g per robot. Elegoo's Rapid PETG is $13.99 per kg. Not PLA: the leaf springs fatigue.

Hall sensor (only if you need it)

The A3144 hall switch is a fallback in case the contact latch sensor proves flaky, and is not in the per-robot price. The bare A3144 is obsolete, so listings vary. If you want the bare part, avoid listings for the "linear hall" version (such as B0FXX97T3F) and KY-003 modules. Ready-made hall modules from M5Stack and Adafruit also exist.