Building a Budget BD-X Droid, Part 2: The Hardware

Without parts there is nothing to build. In this part we order everything for the robot, together. You get one complete shopping list, a short explanation of what every part does, and a checklist for when the boxes arrive.

No electronics experience? No problem. You don’t need to know what a regulator or a bearing is. For every part I tell you what it does, so you know why you’re buying it.

What You’ll Do in This Part

1. Make three small decisions before you order.
2. Order the parts from the shopping list.
3. Gather the tools you need besides the parts.
4. Check your order when it arrives.

This part is about ordering. I haven’t ordered or received anything yet while writing this, so what follows is the plan, built on the project’s official bill of materials. Prices come from that list and may have changed.

Before You Order

Decision 1: spare servos? The robot needs 14 servos. I’m ordering 16, so one broken motor doesn’t stop the project. That’s my suggestion, not part of the official list. It costs €28.

Decision 2: the expression package now or later? The eyes, antennas and speaker come in Part 8. If you order them now, you only have to order once. If you wait, you only buy them once the robot walks. Either way is fine. The package costs about €38, because several parts are only sold in bulk.

Decision 3: the motion sensor. The €40 sensor is the most expensive small part. The project’s author says he saw much cheaper ones but hasn’t tried them. This series uses the one from the list, because it’s the one that’s known to work with the guide.

The Shopping List

The list is grouped by what the part is for, so you can see how the robot fits together. The Shop column says where I plan to order each part. Prices are the ones from the project’s official list, in euros. They are estimates: I haven’t checked what the shops charge today, so look at the product page before you pay.

The list at a glance

Motors and control (€235.66)

Brain and sensors (€80.08)

Power and battery (€51.87)

Cables and mechanical parts (€38.49)

Printing (€20.00)

Optional: the expression package (about €38)

The official list also has a microphone (€15) and a camera (€5). The project hasn’t integrated them yet, so I’m skipping them.

Guides for These Parts

Want to know more about a part before you order it? These guides on this site go through the details:

• Raspberry Pi Zero 2 W, the brain of the robot, and what to check about its header.
• Feetech STS3215 7.4 V servo, the joint servo, including the voltage limit.
• Waveshare bus servo adapter (A), how it connects the Raspberry Pi to the servos.
• Adafruit BNO055 motion sensor, what it measures and how to calibrate it.
• 2S BMS, the battery protection board.
• 2S USB-C charger module, and why it charges at 1.1 A.
• 8A UBEC, the regulator that gives the Raspberry Pi its 5 V.
• MAX98357 amplifier, the little amplifier that drives the speaker.

What You Need Besides the Parts

These aren’t on the official list, and the project doesn’t give prices for them:

• A 3D printer, with PLA and TPU filament.
• A soldering iron and solder, and some wire.
• M3 screws. The guide mentions M3×10 and M3×6, but the total number isn’t in the guide yet. An assortment box with different lengths is a safe choice.
• Blue threadlocker (Loctite 243), for the screws that go into the motors.
• An Xbox One controller to steer the robot.
• A computer to put the software on the SD card.

Check whether the Raspberry Pi you buy has its pin header already soldered on. The wiring uses its GPIO pins, and without a header you would have to solder wires directly to it.

The 3D Printed Parts

Most of the robot is printed. The print guide lists 36 files, which make 51 parts. Print them in standard PLA with 15% infill, except the foot soles (foot_bottom_tpu.stl), which are TPU at 40%.

Don’t print everything at once. Print one foot and one structural joint first, and check that the servos and inserts fit. We do that in the next part.

When the Boxes Arrive

Check everything before you start building. A wrong part now is easier to fix than a wrong part halfway through.

1. Count the servos. You should have 16 large servos and 2 small ones.
   Good result: every servo is the same type.
   Bad result: the number is off. Contact the shop now.
2. Check the label on the servos. They must be the 7.4 V version, also sold as “19 kg·cm”.
   Bad result: a 12 V or “30 kg·cm” label. Those don’t fit this battery. Send them back.
3. Check the battery cells. Two lithium-ion cells of the type you ordered, undamaged.
4. Check the Raspberry Pi and the charger. Make sure the charger really is for two cells in series (2S).
5. Tick off the list. Everything on the shopping list is there, and you noted what you actually paid.

If It Didn’t Work

• A part is out of stock or the price changed. The official list isn’t kept up to date. Look for the same part elsewhere, and check the specs before you buy it.
• A link no longer works. Search for the part by its name and specs from the table.
• You ordered the wrong servo voltage. That’s the most expensive mistake on the list. Return them and order the 7.4 V version.
• Parts from far away take longer. That’s common experience, not a promise: order the parts from Alibaba and AliExpress first.

What You Have Now

A complete shopping list, an order placed (or ready to place), and a checklist for when it arrives. Next up: printing a test foot and a test joint, to make sure everything fits before we print the whole robot.

If a link or price has changed, tell me through the contact page and I’ll update the list.

Words Explained

• Servo: a small motor that turns to an exact position and holds it.
• IMU: a motion sensor that measures tilt and movement.
• Regulator (UBEC): a small board that turns one voltage into another, here the battery’s voltage into 5 V.
• BMS: a battery management system, a protection board for the battery.
• 2S: two battery cells in series, which gives twice the voltage of one cell.
• Heat-set insert: a metal nut you melt into plastic to get a strong screw thread.
• Bearing: a ring that lets a part turn smoothly.
• PLA and TPU: two kinds of 3D printing plastic. PLA is stiff, TPU is flexible.
• Infill: how solid the inside of a printed part is.

Conclusion: Time to Shop

One list, about €426 for the robot, and a clear idea of what every part is for. Order, wait, check, and then we start printing.

Small droid, big plans. Happy hacking!

Unofficial fan project

BD-X and Star Wars are trademarks of Disney/Lucasfilm. This site is not affiliated with, endorsed by or sponsored by Disney. The robot in this series is based on Open Duck Mini by Antoine Pirrone and contributors, released under the Apache-2.0 license.