Bill of Materials
Everything you need to order to build one gadget. Build at least two — a single device on its own does nothing useful.
Parts overview
Off-the-shelf modules. No PCB to fabricate, no custom electronics.
| Part | What it is | Approx. price each |
|---|---|---|
| Waveshare RP2350-Touch-LCD-3.5 | 3.5" capacitive touch + RP2350 microcontroller + onboard battery management + onboard MicroSD slot. The brain and the screen, pre-assembled. | $25 |
| GM861XS QR scanner | Tiny UART QR scanner (~17 mm round, 5 mm deep). Reads QR codes on the other gadget's screen. | $12 |
| 3.3V SPI MicroSD breakout | Bare 6-pin passive board with a card socket. Used for the "guest" card slot during key exchange. | $0.50 |
| RS PRO 3.7V 1800 mAh LiPo | Single-cell lithium polymer battery, ~54 × 35 × 10 mm. | $21 |
| MX1.25 2-pin cable | Connects the battery to the JST socket on the Waveshare board. | $1.30 |
| 2× MicroSD card, 32 GB Class 10 | One for your own card, one for the guest slot of whoever you pair with. | $5–$10 |
Per-device total: roughly USD 70–80 at retail (each gadget uses two MicroSD cards). For a pair, double it.
Where to buy
| Supplier | Parts |
|---|---|
| waveshare.com | Waveshare RP2350-Touch-LCD-3.5 |
| rs-online.com | RS PRO 1800 mAh LiPo (other suppliers also stock equivalents) |
| aliexpress.com | GM861XS QR scanner, 3.3V SPI MicroSD breakout, MX1.25 cable |
| Any reputable retailer | MicroSD cards — SanDisk, Kingston, Samsung, Intenso, all fine |
Direct purchase links for each part are in the order checklist in the public repo.
The "do NOT use" warning
Two of the SD card modules sold on AliExpress look almost identical to the one we use but are incompatible. Picking the wrong one costs half a day of debugging.
Avoid: any MicroSD SPI module with a level converter or an onboard regulator (AMS1117, "5V/3.3V" labelled). These need 4.5–5.5 V on VCC, but the gadget's header only exposes 3.3 V on battery power. The card will fail init with OSError: timeout waiting for v2 card after ~50 seconds.
Look for: an SD module that explicitly says "3.3V only" (not "5V/3.3V"), is small (~18 × 18 mm), has exactly 6 pins (CS, SCK, MOSI, MISO, VCC, GND), and shows no chips on the board — just the card socket and the pin header.
Order checklist
Before you order, make sure you also have the bench tools — they aren't in the parts list.
Order everything in one go. The Waveshare board and the SD module ship from China and are the long pole — two to three weeks is normal.
Next: Hardware — what each module does and how they're wired together.