Bill of materials (BOM) — 8-DoF quadruped#

What you need to build the chapter 13 robot. This is the same configuration as the book’s robot; prices are as of August 2026. The shops listed are in Japan (prices in yen, tax included); outside Japan, look for the same parts by model number.

Electronics #

#	Item	Shop code	Unit price	Qty	Subtotal
1	FEETECH serial bus servo STS3215 12V 30kg·cm	Akizuki 130969	¥3,580	8	¥28,640
2	Serial bus servo driver board (Waveshare 25514)	Akizuki 131227	¥1,280	1	¥1,280
3	Switching AC adapter 12V 5A AD-A120P500	Akizuki 110663	¥2,380	1	¥2,380
4	Breadboard jumper wires (male–female) 15 cm	Akizuki 108932 etc.	¥280	1	¥280
5	USB Type-C cable (must support data): C-to-C — CIO SL30000-CC / A-to-C — CIO SL30000-AC	—	from ¥1,500	1	¥1,500
6	ROBOTIS 3P Extension PCB (bus splitter, optional)	903-0142-000	¥495	2	¥990

You need only one of the two cables: C-to-C if your computer has a Type-C port, A-to-C if it only has Type-A.

Total about ¥35,100. The servos are 81% of it.

The screws for assembly (M3×6 ×40, M2×6 ×16) come with the STS3215 servos, so you do not need to buy them.

You also need an Arduino UNO R4 WiFi. The AC adapter above is enough to walk the robot on the floor (the book’s robot walks with the adapter plugged in). Use “Battery power (optional)” below only if you want to walk without a cable.

AC adapter capacity #

From the datasheet’s stall torque 2.942 N·m / 2.7 A (stall: the shaft is held by the load and cannot turn, which draws the largest current), assuming current is roughly proportional to torque, we estimated 0.92 A/(N·m).

State	Torque per joint	Total for 8
Holding the standing pose (measured 0.27 N·m)	9%	about 2 A
Walking (rms 0.75 N·m, fastest gait; a gait is the pattern of leg movements)	76%	about 5.5 A
All 8 stalled at once	100%	21.6 A

We chose 5 A. Bench work (calibration, identification, holding a pose; identification means measuring the real robot to fit the simulation’s parameters) needs about 2 A, and walking fits on average. The peaks at each footstep exceed 5 A, so adding one electrolytic capacitor (2200–4700 µF / 25 V) across the terminal block reduces servo resets caused by the voltage sagging.

There is no point in buying a 21.6 A supply: every joint stalls at once only in an accident.

When walking on the AC adapter, hold the DC cable slack in your hand so the robot never pulls on it. Dragging a 1 m DC cable, the cable has actually come out.

The driver board’s input limit is 12.6 V. A fully charged 3S LiPo (three lithium-polymer cells in series) is exactly 12.6 V, right at the limit.

Battery power (optional)#

Use a battery only if you want to walk without a cable. Taking 12 V from a USB-C PD power bank is the easiest way, and it works on the real robot.

#	Item	Requirement
a	USB-C PD power bank	Its output PDOs (the voltage/current combinations a USB PD charger can supply) must list `12V ⎓ 3A`
b	USB Type-C to C cable (short)	Supports data. A normal 60 W cable is fine for 3 A
c	PD trigger cable PDC-12VE	Fixed 12 V, 5 A, DC 5.5/2.1, centre positive, ¥1,830

No soldering or crimping: plug power bank → C-to-C cable → trigger cable → the board’s DC jack.

Choose by requirement, not by product name. Power banks change quickly and the models below will disappear within a few years. There is only one test: does the product page list 12V among its output specs? “PD 65W” or “up to 100W” alone does not tell you.

12 V is optional in USB PD

USB PD’s fixed voltages are basically 5 V / 9 V / 15 V / 20 V; 12 V is optional, and plenty of products do not have it. Anker does not offer 12 V on its USB-C outputs (checked in the official specs: the Nano Power Bank 10K 45W has 5V/9V/10V/15V/20V, and 12 V is only available on the USB-A port’s QC (Quick Charge) modes).

Products with only PPS (Programmable Power Supply, a USB PD mode whose voltage can be set in fine steps, e.g. 3.3–11 V) cannot deliver 12 V to an ordinary fixed-12 V trigger cable either.

And 15 V and 20 V exceed the board’s 12.6 V limit. If you use a switchable trigger cable, do not set it to 15 V by mistake; a fixed one is safer.

For reference, products whose official specs confirm 12 V / 3 A (as of August 2026; examples only):

Product	Capacity	Weight	12 V
CIO SMARTCOBY TRIO 67W	20000 mAh	about 333 g	3 A
CIO SMARTCOBY Pro PLUGⅡ 67W3C	5000 mAh	about 308 g	3 A
ELECOM DE-C81L-10000	10000 mAh	about 220 g	3 A

Light models that do have 12 V stop at 1.6–2.5 A. We could not find a 12 V / 3 A model in the 100 g class: 12 V / 3 A needs a 45 W-class unit or bigger, and its circuitry and cells cannot get below 200 g.

Always check the polarity. PD trigger cables with centre negative are sold for guitar effects pedals and look identical; plugging one in reverses the polarity instantly.

12 V / 3 A = 36 W leaves no margin, so add one electrolytic capacitor (2200–4700 µF / 25 V) to absorb inrush current. If it still drops out, move up to a 67 W class unit.

Warning

Electrolytic capacitors have polarity (+ and −). Put the longer leg on DC+ and the leg on the side of the stripe (−) on DC−, screwed into the driver board’s terminal block (the green screw terminal next to the DC jack). Fitted backwards, it can burst. It is not in the parts list above, so get one separately.

Things to know before wiring #

This board’s UART (serial link) is wired RX to RX and TX to TX (the board’s RX to the Arduino’s RX, TX to TX). Note that this is not the usual crossed RX/TX UART wiring (stated on Waveshare’s product page).

The board has a USB Type-C port and an A/B jumper to switch between “driven from the PC over USB” and “driven from the Arduino over UART”. Bring-up and calibration are quicker over USB, so it is worth setting up both.

Three jumper wires between the Arduino and the board (the board’s UART header ↔ D0/D1/GND) are enough. If one comes out while walking the robot falls over, so for a firmer connection make your own with 2.54 mm pin headers and sockets.

The driver board. The yellow part is the A/B jumper; USB Type-C and the DC jack are at the bottom right#

Splitting the bus (optional)#

Servos can be daisy-chained (each servo has two 3-pin connectors), so a hub is not required. But the quadruped’s legs come out at four corners, so a splitter board keeps the cables short and makes routing easier.

FEETECH’s connector is the Molex Mini-SPOX 5264 (2.50 mm pitch, 1 = GND / 2 = Vcc / 3 = signal), the same as Dynamixel’s Molex line (AX / MX), so ROBOTIS’s splitter boards work as they are.

Product	Part no.	Connector	Price (tax incl.)	For this robot
3P Extension PCB	903-0142-000	Molex 3P (header 22-03-5035)	¥495	This one. 6 ports, M2 screw holes
3P JST Expansion Board	903-0306-000	JST 3P	¥1,584	Different family
4P JST Expansion Board	903-0307-000	JST 4P	¥1,716	For RS-485

Do not mix up the Molex and JST versions. ROBOTIS itself says the Molex version is “not compatible with the X series, which uses JST connectors” and the JST version “not compatible with the AX / MX, which use Molex connectors”. FEETECH is on the Molex side.

The header 22-03-5035 is from the Mini-SPOX 5267 series, the proper mating part for the 5264 housing. However, we have not confirmed an actual case of a FEETECH cable plugged into the ROBOTIS board. At ¥495, the quickest way is to buy one and check.

Buy two and split them between the left and right legs. The Mini-SPOX 5264 is rated 3.0 A per contact, and the total while walking is about 5.5 A (table above), so all 8 servos through one hub exceeds the rating. Split left and right, each carries 4 servos, about 2.7 A.

The back of the board is not insulated. Mounting it on the plastic body is fine, but do not put it on metal (ROBOTIS’s warning).

Akizuki Denshi and Switch Science do not carry an equivalent splitter board (as far as we searched, they only have bare connectors and housings, or I²C / USB hubs).

3D-printed parts #

Download the STLs below (in millimetres; exported from the same CAD data as the 3D viewer in Assembly).

Part	File	Size [mm]	Qty	Volume
Body	`body.stl`	230.0 × 110.0 × 50.0	1	244.3 cm³
Leg bracket	`bracket_outline.stl`	127.9 × 52.2 × 12.0	4	19.1 cm³
Leg link	`leg_link1.stl`	80.0 × 52.0 × 26.0	4	39.2 cm³

body.stl (×1)

bracket_outline.stl (×4)

leg_link1.stl (×4)

Sliced projects (for Bambu Studio)#

Project files with the same settings and layout as the book’s robot was printed with (Bambu Lab X1C, 0.4 nozzle, 0.28 mm, ABS, automatic tree supports, Textured PEI plate). To print in PLA, just change the filament.

Open in Bambu Studio launches Bambu Studio and opens the project directly (Bambu Studio must be installed). Because the file does not come from MakerWorld, Bambu Studio asks “This file is not from a trusted site”; click Yes to open it.

body.3mf (body plate)

body plate

leg_parts.3mf (plate with the 8 leg parts)

leg plate

9 parts, about 477 cm³ in total.

../_images/quadruped_parts.jpg — The 9 printed parts and 8 STS3215 servos#

The left and right legs use the same parts. Print each bracket and link STL four times; there are no separate left and right versions.

Printing them yourself #


Material	PLA is enough. PETG or ABS also work (ABS warps easily, so use a printer with an enclosure)
Bed	At least 230 mm on one side. The body is 230 mm long
Filament	About 240 g (estimate; slicing gives the exact figure)
Cost	About ¥700 (roughly a quarter of a 1 kg spool of PLA at ¥2,500–3,500)

The body is 230 mm, so check the bed size before you buy a printer.

Printer	Bed	Body
Bambu X1C / P1S	256 × 256	Fits
Prusa MK4	250 × 210	Fits
Bambu A1 mini	180 × 180	Does not fit

Plan on one job for the body and one for the 4 brackets + 4 links. The bracket is a 12 mm plate, so print it flat (it takes the servo’s torque, and printing flat puts the layers in-plane, which is also the right choice for strength).

PLA’s default slicer profile (0.20 mm layers) is enough. To make parts stronger, adding walls (2 → 4–6) helps more than raising the infill.

Ordering prints #

For when you have no printer, the bed is smaller than 230 mm, or you want the toughness of nylon.

Service	Process	Typical price	For 477 cm³
PCBWay Shared Projects	FDM / SLA / MJF and more	Cheap because FDM is available	Quote needed
DMM.make 3D print	MJF PA12	from ¥113/cm³	from about ¥54,000
DMM.make	SLS PP	from ¥63/cm³	from about ¥30,000

Ordering prints costs more than the servos. Printing yourself costs ¥700, a 77× difference. A printer that can print 230 mm costs ¥40,000–90,000, so one order costs about as much as a printer.

And you will always rebuild a robot’s frame: a leg breaks, you move a hole, you change a mounting angle. Designing on the assumption that every rebuild costs ¥54,000 is not realistic. Printing yourself is the main route; ordering is the exception.

The mass differs from the simulation model #

The mass of printed parts varies a lot with material and infill (477 cm³ printed solid in PLA is about 590 g; at 20% infill, about 240 g). It will always differ from the simulation model, so if you want to compare with the simulation, weigh the parts you made and correct the model’s masses. Inertia and the centre of mass change too.

License #

The STL, 3MF, CAD, URDF, firmware and trained walking policy distributed on this page are under the Apache License 2.0.

Commercial use is allowed. You may sell them as a kit, with no prior permission or notice required.
Neither the author nor the University of Tokyo will ask for license or royalty fees.
We will not ask for exclusive arrangements either. Anyone can build it and anyone can sell it.
You may modify and redistribute them. Follow the Apache-2.0 terms (keep the copyright notice and LICENSE, and state your changes).

Trademarks are not covered by the license. Stating facts such as “for this book” or “SO-101 compatible” is fine, but please ask first before using labels such as “official”, “University of Tokyo” or “supervised by the author”.

To businesses selling kits

Once you start selling, let us know through an Issue or by e-mail and we will list you in a “buy a kit” section on this page. Listing is free, with no listing or referral fees. Kits confirmed to work on the real robot will be marked as such.

Sources #

Akizuki Denshi product pages (linked above)
Bus Servo Adapter (A) — Waveshare (RX-RX / TX-TX, 9–12.6 V)
3P Extension PCB — ROBOTIS Japan e-shop (price, stock, M2, header 22-03-5035, uninsulated back)
USB interface / hub boards — ROBOTIS Japan e-shop (comparison of 4 hub boards)
How to Use STS3215 — Cirkit Designer (5264-3P, 1 = GND / 2 = Vcc / 3 = signal)
Molex Mini-SPOX (5264) Connector Guide — Keszoox (3.0 A / 2.50 mm / friction lock)
PKT shop PDC-12VE — Sengoku Densho (fixed 12 V, 5 A, centre positive)
Anker Nano Power Bank (10K, 45W) (confirms no 12 V in the USB-C PDOs)
PDOs of the CIO / ELECOM power banks: each official product page (linked above)
DMM.make 3D print (price per cm³ by material, build size)
Dimensions and volumes measured from the distributed STLs