Parts and prices

Every bought part for one Tailwhip is in the tables below, including the parts many builders already have (marked "Often already in a robotics parts drawer"), so that you can build one from this page. The list is AI-generated and unbuilt: check each listing before you order.

What it costs

The table is split into three groups, because a single total mixes up what the robot needs with what is optional and what is a workshop tool.

"Per robot" uses the per-unit price inside each pack. Most parts come in packs of 2 to 10, so buying every pack new for a single robot costs far more up front (about $250 for the core packs) but leaves the parts for several more robots: it is the right way to think about a class or a club, not one build.

Compared with Ouro ($87): the robot itself is cheaper, mostly because two $3 TT gearmotors and two $2.50 SG-90s replace eight MG90S servos. What both robots share sets the floor: two ESP32s (about $14), a battery and its charger.

Prices change. Check each listing before you order.

Where it departs from the SPEC's parts list

The 18650 cells

Use genuine high-drain cells from a battery specialist, as a matched pair (same model, bought together). A Molicel P28A (2800 mAh, 35 A continuous, $5.99 [D]) is the first choice; Samsung 25R, Samsung 30Q and Sony/Murata VTC6 are all good. Two reasons:

• Low internal resistance. The motors pull short bursts of 1-3 A [E]. A good cell sags only a few tens of millivolts at that; a cheap one sags enough to change how hard the wheels push from one second to the next.
• A real rating. A 15-35 A continuous rating gives a large margin over the robot's 4 A worst case [E].

Buy them from 18650batterystore.com, imrbatteries.com, liionwholesale.com or Illumn, not from Amazon or eBay, where cells are often counterfeit or rewrapped. Never buy "UltraFire", "GTF" or anything claiming 9,900 mAh.

Tools and consumables

Not counted in the price:

• Multimeter with a 10 A current range: to set both Mini360 bucks to 5.0 V before anything is connected, and for the bench measurements
• Soldering iron and solder, flush cutters, wire strippers, small Phillips screwdriver, 2.5 mm hex key, calipers
• Kitchen scale (mass, centre of mass, stall torque with a lever)
• A board you can tilt, covered with your demo surface (tyre friction)
• Phone with slow-motion video (servo speed)
• Two USB data cables (one per ESP32)
• CA glue for the encoder rings; a strip of bicycle inner tube for the tail foot
• Ender 3 or any FDM printer with a 220 x 220 mm bed, PLA and ABS

Core robot: about $62.15 per robot

Everything the robot needs to rear up and balance.

Optional: about $21.98 per robot

The camera head (milestone 6) and spare metal-gear servos.

Shop tools: about $29.97 per robot

Bought once, used for many robots. Skip what you already have.

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.

Many of these parts are common in a robotics parts drawer. They are all listed so that you can build a Tailwhip from this page; if you already have them, only the items marked BUY are new.

Boards: ESP32, IMU, motor driver, camera

• ESP32: classic ESP32-WROOM-32, not C3, S2 or S3. You need two: one on the robot, one plugged into the laptop as the USB radio bridge. 30-pin and 38-pin DevKits both fit; the robot board sits on the spine with its header pins pointing back. Adafruit's HUZZAH32 Feather ($19.95) is genuine but a different shape.
• IMU: GY-521 (MPU-6050). Cheap 5-packs are fine. Adafruit's MPU-6050 ($12.95) is a genuine sensor on a different board, so the mount would need changing.
• Motor driver: MX1508 module, 2 to 10 V, two inputs per motor and no enable pins. The chip's datasheet rates one channel at 1.5 A continuous and the other at only 0.8 A continuous, and modules rarely say which is which. A TT motor can pull 1.1 to 1.5 A when stalled, so do not hold the robot against a wall with the motors on.
• ESP32-CAM: the AI-Thinker style board with the OV2640 camera. Only needed at stage 4. A kit with the small USB programmer board (ESP32-CAM-MB) saves wiring up a separate USB-serial adapter.

Motors, wheels and servos

• TT motor: the common yellow 1:48 gearmotor with a straight (not 90-degree) shaft, 3 to 6 V. Most listings bundle a 65 mm wheel with each motor. Adafruit sells the motor at $2.95 and 65 mm wheels at $1.50.
• Wheels: 65 mm with a rubber tyre. Hard plastic tyres slip on smooth floors.
• SG-90 servos: must say 180 degrees. A 360-degree or "continuous rotation" SG-90 looks identical and will not hold a position.
• MG90S servos (buy two): the same body as the SG-90 with metal gears, so one drops straight into the tail mount if the tail SG-90 strips its gears. Again, the listing must say 180 degrees. Waveshare sells them at $2.59 each; Adafruit's MG90D ($9.95) is the trustworthy option.

Sensors and regulators

• IR sensor: the TCRT5000 module with an LM393 comparator and a blue trim pot, with a digital "DO" output. Power it from 3.3 V so its output is safe for the ESP32.
• Mini360 buck: the tiny adjustable board, 4.75 to 23 V in. Most listings rate it 1.8 A continuous and 3 A surge. It comes set to a random voltage: set it to 5.0 V with a meter before you connect anything.

Battery cells (18650)

• Buy cells from a battery specialist, not Amazon or eBay. Cell listings on the big marketplaces are often counterfeit or rewrapped cheap cells. Good shops: 18650batterystore.com, imrbatteries.com, liionwholesale.com, Illumn.
• Never buy "UltraFire", "GTF", or anything claiming 9900 mAh. No real 18650 holds more than about 3,500 mAh.
• Pick a genuine high-drain cell. Good choices, in stock at 18650batterystore.com when checked: Molicel P28A ($5.99, 35 A continuous), Samsung 25R ($4.99, 20 A), Samsung 30Q ($6.99, 15 A) and Sony/Murata VTC6 ($7.99, 15 A). The P28A is also $5.99 at imrbatteries.com.
• Why high-drain: these cells have low internal resistance, so the voltage barely sags when the robot pulls 1 to 5 A. The ESP32 is less likely to brown out when the motors kick. A real 15 to 20 A continuous rating also leaves a big margin over the robot's few amps.
• Flat top, unprotected. The BMS (item 13) protects the pack, so you do not need protected cells, which are also longer and may not fit. Most spring holders take flat tops; buy button tops only if your holder needs them.
• Buy a matched pair: same model, same shop, bought together. Charge both full before you build the pack.

Battery holder

• 2 x 18650 in series, two leads out, about 41 x 77.5 x 20.5 mm outside (what the CAD assumes). SparkFun's PRT-12900 ($1.95) is 74.5 x 40.8 x 17.7 mm but does not say series or parallel: with cells in, the leads should read 7 to 8.4 V.
• The BMS also needs the middle of the pack (between the two cells). Solder a thin wire to the strap that joins the cells, or pick a holder where that strap is reachable.

Battery protection board (BMS)

• 2S, 7.4 V nominal / 8.4 V full, at least 5 A continuous. The robot pulls up to about 4.5 A for a second.
• Balancing or not. A balanced board bleeds a little current from the higher cell near full charge, so the two cells stay matched over months of charging through the robot. That is why the table picks one. An unbalanced board is smaller but lets the cells drift; if you use one, check each cell with a meter or a smart charger every few weeks.
• The listed board: HX-2S-JH20 from ShillehTek, $6.99. 10 A continuous, about 15 A for short bursts; over-charge cutoff 4.25 to 4.35 V per cell, over-discharge cutoff 2.4 to 2.6 V per cell; passive balancing at about 40 mA. It is about 50 x 22 x 3.5 mm, larger than the 30 x 16 mm slot in the CAD, so the slot has to grow.
• Cheap unbalanced "2S 5A" boards are typically 5 A working, 7 A trip, over-discharge 2.5 to 3.0 V. Check the over-current figure: anything that trips below about 6 A will cut out when both motors start at once.
• After wiring, a BMS may stay "locked" with no output until a charger is plugged in once.

Fuse and power switch

• Polyfuse: radial PTC, 3 A hold, 6 A trip, 16 V or more: Littelfuse RUEF300 ($0.62 at DigiKey) or Bourns MF-R300.
• The power switch carries all the motor current: 3 to 4 A peaks and 1 to 1.5 A on average. Tiny SS12D00-style slide switches are rated only about 0.3 to 0.5 A and will burn out. Do not use one. Also beware: in some slide-switch part numbers "3A" is the actuator length (3 mm), not the current rating. Read the rating in the spec table.
• First choice: Pololu Mini MOSFET Slide Switch LV, item 2810 ($4.49). A tiny slide switch turns a MOSFET on and off, so the switch itself carries no current. Good for 1.8 to 16 V, 3 A continuous with only a 55 °C rise, and it protects against a reversed battery. The board is 15.2 x 15.2 mm; the lever sticks out about 1 mm.
• Second choice: KCD11 mini rocker, rated 3 A 250 VAC / 6 A 125 VAC. It snaps into a 10 x 15 mm hole and is about 18 mm deep behind the panel.
• Either one is bigger than the 6.2 x 10 x 6 mm slide switch the CAD currently assumes.

Charging

• Charger: a 2S Li-ion charger, 8.4 V, 1 A, CC/CV, with a 5.5 x 2.1 mm plug. Not 7.4 V, not 9 V, and not a 12 V adapter. Check the plug size (some are 5.5 x 2.5 mm) and that the centre is positive. A red/green LED that turns green when full is handy.
• Charge jack: a 5.5 x 2.1 mm female jack with short wires, connected to the BMS P+ and P-. Check which wire is centre positive with a meter before soldering.
• Smart charger (optional): an XTAR VC2S ($14.99 at imrbatteries.com) or Nitecore i2 charges cells one at a time and shows each cell's voltage, which is the easy way to check that the two cells still match.

Small electronics

• Capacitor: 470 µF, 16 V (or 10 V), low ESR, across the servo 5 V rail. Mind the polarity stripe.
• Resistors: 100 kΩ and 33 kΩ for the battery divider, plus two more 100 kΩ as IR pull-downs. Check that any kit lists 33 kΩ; the Aniann 1280-piece kit does, smaller kits may not.
• 100 nF ceramic capacitor (marked 104) across the divider.
• JST-XH 2-pin pairs on the motor leads, so the motors unplug. Pre-wired pairs (Adafruit, $0.95 a pair) save buying a crimp tool. JST-XH is really 2.5 mm pitch.
• Header pins and perfboard: 2.54 mm male header for the two servo plugs on a small perfboard. A 4 x 6 cm board cut down works.
• Dupont jumpers: female-female, 10 cm, pushed onto the ESP32 pins.

Screws, wire, ties and filament

• M3 x 70 mm socket head, two, clamping both motors through the core. Partly threaded is fine. M3 threaded rod cut to length works too.
• M3 assortment kit with M3 x 8, M3 x 10 and nuts: one kit covers the rest of the screws.
• Tail pivot: an M3 shoulder screw with a 4 mm shoulder, 8 mm long (search "shoulder bolt" or "ISO 7379 M3"; McMaster, Misumi and Zoro all stock it). Fallback: an M3 x 16 screw through an 8 mm long, 4 mm brass spacer.
• Wire: silicone 22 AWG red and black for battery, BMS, switch, motor and buck wiring (about 0.6 m); silicone 26 AWG in a few colours for signals (about 1.5 m).
• Zip ties 2.5 x 100 mm, and a heat-shrink assortment.
• Filament: about 92 g of PLA for the body and 10 g of ABS for the tail, which has to flex.