The velocity sensor with nothing to hide.

Most VBT devices report a velocity number with no way to check how it was computed. On ODKI‑VBT, the entire rep‑detection pipeline — sensor fusion, drift correction, rep scoring — is documented chapter by chapter, and the Bluetooth protocol it speaks is published under a public‑domain license. You're not asked to trust the number; you can go and read exactly how it's produced.

The firmware never force‑resets the integrated velocity signal, which is the usual source of jumps and glitches in cheaper designs. Instead, brief moments of genuine stillness between reps are used to measure exactly how far the signal has drifted, and that drift is corrected retroactively across the interval it built up over. Every completed rep also ships with its own data‑quality score, so you can see how clean a given reading is rather than just trusting it blindly.

Yes. The BLE protocol is released under CC0 (public domain), separately from the firmware and hardware licenses. Any developer can build a third‑party app against it without asking ODKI for permission — the official app is one option, not a requirement.

Connecting to the sensor and viewing live data is free. Some extra features — long‑term training‑data management and AI‑based analysis — are paid add‑ons on top of the app. Either way, this has no effect on the sensor itself, which works over the open protocol regardless of which app you use.

Three small neodymium disc magnets (~2kg pull force each) hold the sensor directly onto a standard steel barbell — no strap, no mechanical clip. It attaches and detaches in seconds and stays put through a working set.

A 400mAh Li‑Po cell, charged over USB‑C. Battery percentage is self‑calibrating and reported live over Bluetooth, so the companion app always shows an accurate charge level rather than a rough voltage guess.

Yes — firmware updates ship over‑the‑air via Bluetooth DFU. The app checks the sensor's current version automatically and offers a new build when one is available, no cables or manual flashing required.

The reference design is built around an off‑the‑shelf Seeed XIAO nRF52840 Sense module, a small Li‑Po battery, three magnets, and a power switch — a published bill of materials with sourcing notes puts total parts cost roughly in the €20–60 range, depending mostly on where you source the microcontroller. You flash the source‑available firmware with the Arduino IDE and build (or 3D‑print) an enclosure. Full instructions live in the GitHub repository.

No, selling isn't allowed — but it's worth being precise about what the licenses actually say, not just the bottom line. The hardware design is CC BY‑NC‑SA 4.0: you can build, modify, and share it for non‑commercial use, provided you credit the project (Attribution) and release any modified files you share under this exact same license (ShareAlike) — so improvements can flow back to everyone. The firmware is GPL‑3.0 modified by the Commons Clause: you can run, study, modify, and redistribute it non‑commercially, but any copy you share — modified or not — must stay under the same license with its notices intact, and neither it nor a modified version of it may be sold. In both cases, selling an assembled unit — under the ODKI name or otherwise, built from these files or a derivative of them — is reserved to ODKI. Neither license is OSI‑approved "open source" in the strict sense (that's specifically what the Commons Clause and the NonCommercial term remove) — the project calls this "source‑available" instead, deliberately. Full legal text: LICENSE-HARDWARE and LICENSE-FIRMWARE in the repository. The Bluetooth protocol spec is the one part of this project under a fully open, unrestricted license — CC0, public domain, no conditions at all.

Yes. A factory ODKI‑VBT already comes with the case included in the €129 price — nothing extra to buy. If you built your own sensor from the published files, the same ODKI‑branded case is available on its own for €19.

The sensor streams data directly over Bluetooth to whichever app you're using — no ODKI account is required to use the sensor itself. What happens to that data afterwards is up to the app you choose, official or third‑party.