Buy the parts
The bill of materials, with prices, sources and the date each price was checked, lives in the repository at
hardware/bom.md. This page is the how: what to check before you order, what you can substitute and what you
must not.
Before you order: checklist
- [ ] Read the whole stage first (for example the H1 tutorial), so you know what each part is for.
- [ ] Re-check prices and stock. Every price in the BOM has a "checked on" date; anything older than 3 months is a guess. Single-board computer prices rose several times in 2025–2026.
- [ ] Check lead times. RTK GNSS boards and genuine modules can be weeks out; order those first.
- [ ] For H2: check the mower's revision against the OpenMower compatibility list before buying a used mower or a mainboard. The wrong carrier-board revision does not fit.
- [ ] Region: chargers and the dock supply must match your mains (US 120 V / EU 230 V); check the plug and the rating on the label, not the listing.
- [ ] Buy batteries only as complete packs with a BMS from a seller who ships them legally (UN38.3 paperwork). Don't build a pack.
- [ ] Keep every receipt and order number: they go into the build record (part lots).
What you can substitute
| Part | Substitute freely? | Rule |
|---|---|---|
| Raspberry Pi 5 (8 GB) | With care | 4 GB works for H1; the 8 GB leaves room for mapping and bags. Must be a Pi 5 (arm64, same container). |
| NVMe SSD, cooler, PSU | Yes | Any reputable brand. The PSU must be the 27 W USB-C type (5 V 5 A); weaker supplies throttle the Pi. |
| RTK GNSS receiver | No: genuine u-blox ZED-F9P | Clone or "compatible" F9P boards are common and fail RTK fix in ways that look like software bugs. Buy from the board maker or an authorised distributor. |
| GNSS antenna | Only multiband (L1/L2) | A single-band antenna works but RTK takes far longer to fix and drops more often. |
| IMU (BNO085) | With care | Another 9-DoF IMU needs its own driver and a new noise calibration; the sensor report must pass again. |
| Fuses, fuse holders | No | Exactly the rating in the wiring drawing. A bigger fuse is not a safer fuse. |
| STOP button, interlock relay / MOSFETs | No | Part of the safety chain: the exact part, or a re-run of every safety test at gate 3. |
| Blade and blade bolts | No | The mower maker's genuine blades only. Never a sharpened replacement or a "universal" blade. |
| Battery pack | No (H2) / with care (H4) | H2 uses the mower's own pack. H4: a 7S pack with BMS, about 150 Wh, from a reputable maker. |
| Cable, connectors, heat-shrink | Yes | Same or larger wire gauge; never smaller. |
Counterfeits
The parts most often faked are GNSS modules, IMUs, fuses and battery cells. If a price is far below every other seller's, assume it's a fake. The receiving inspection has the signs to look for.
Ordering in batches (for more than one robot)
Order safety-chain parts (STOP buttons, fuses, interlock parts) from one lot for a batch of robots, and record the lot number. If a lot turns out to be bad, the build record tells you exactly which robots have it.