Build on it
A barbell sensor from a handful of parts on a Seeed XIAO nRF52840 Sense, with a CC0 protocol you can build on.
- Full BOM and CAD
- Arduino firmware with docs
- Byte-exact Bluetooth spec
- Python log viewer and replay tool
Velocity-based training sensor · Source-available · CC0 Bluetooth protocol
The velocity sensor with nothing to hide.
ODKI-VBT holds onto a steel barbell with magnets and shows bar speed, displacement and reps on your phone, live. The algorithm, the hardware files and the Bluetooth protocol are public, so you can check how every number is made.
Founders case €19: 40 cases for self-builders, orders open early Novemberorders open now, until 30 November 2026 or until sold out. Ready-made sensor + case €129: free pre-order, no payment taken.
Firmwarev3.11.31 ↗
Sampling100 Hz fusion · 20 Hz live stream
AlgorithmDocumented end to end ↗
AccuracyValidation in progress →
See it work
What you see on your phone.
Screenshots from the free companion app. A video of a full set, start to finish, is coming.
Why ODKI
You shouldn't have to trust a number you can't check.
ODKI-VBT publishes the whole chain: the sensor fusion, the drift correction, the rep detection, the Bluetooth protocol. Read it, build it, or plug your own app into it. If a number looks wrong, you can find out why.
01
Readable
The algorithm is documented stage by stage.
02
Buildable
BOM, CAD and firmware are on GitHub, source-available for non-commercial use.
03
Unlocked
The Bluetooth protocol is CC0: any app can connect, and your data never needs our cloud.
Source-available, not OSI open source. Exactly what that means →
How it works
From raw motion to a number you can check.
Four stages, each documented in the firmware docs.
01 / SENSE
Read the motion
The IMU is read at 416 Hz and averaged to 100 Hz. Orientation uses a median-filtered signal; velocity uses the unfiltered one, so real impacts aren't erased.
02 / ORIENT
Fix the orientation
A Madgwick filter fuses accelerometer and gyroscope into a stable 3D orientation.
03 / INTEGRATE
Correct the drift
Velocity is never force-reset: drift is measured in moments of stillness, then removed retroactively.
04 / STREAM
Report the rep
Peak and mean velocity, acceleration and a signal-consistency score, pushed live over Bluetooth.
Any set can be logged at 100 Hz over USB and replayed through the real tracking engine with the tools in the repo.
Datasheet
- Core
- nRF52840 + LSM6DS3TR-C 6-axis IMU (Seeed XIAO nRF52840 Sense)
- Firmware
- v3.11.31, updates over the air
- Connectivity
- Bluetooth Low Energy, CC0 protocol spec
- Sampling
- 100 Hz fusion loop, IMU read at 416 Hz
- Live stream
- 20 Hz over Bluetooth
- Measures
- Vertical axis only (world frame): velocity, acceleration, displacement per rep
- Mounting
- 3 neodymium magnets in the shell: steel bars only
- Battery
- 400 mAh Li-Po, USB-C. Battery life not measured yet
- Licences
- Firmware GPL-3.0 + Commons Clause · Hardware CC BY-NC-SA 4.0 · BLE protocol CC0
Validation, in public
We haven't published an accuracy number yet. Here's how we'll get one.
ODKI-VBT is being compared with a linear encoder, itself checked against a tape measure before use. The method is public before the results; the raw data will be published whatever it shows.
The method
A checked reference
A linear encoder, checked before use. Two sensors on the same bar, back squat and bench press from light to heavy loads.
What we'll publish
Everything
Paired raw data, scripts and firmware version, with Bland–Altman limits of agreement, ICC, SEM, CV and mean absolute error. Including where it does badly.
Status
Not yet validated
Today it's a well-documented sensor that hasn't been benchmarked yet. Co-validation with a university lab is planned for 2027.
Every rep already carries a signal-consistency score, so an inconsistent reading is flagged, not hidden. It's a self-check, not an accuracy estimate.
Read the validation plan Run a lab with an encoder? Co-validate it with us
Who it's for
Four ways people use it.
Coach on evidence
Bar speed for every rep, live on your phone, so you can adjust load and end sets on evidence, not feel.
- Magnetic mount on steel bars, seconds to attach
- Free app for pairing and tracking
- See exactly how the number is computed
Not yet validated: we want to hear from coaches who'll tell us what's wrong.
Audit it
A VBT sensor you can audit: raw per-sample logs, a byte-exact protocol, a documented algorithm.
- Published validation method
- Results of joint work published openly
Train by speed
Track bar speed like pace or heart rate: see when a set is slowing down before it grinds to a stop.
- No subscription to use the sensor
- Your data stays on your phone
Choose your way in
Build it, finish it, or let me build it.
Same firmware, same open protocol, whichever you choose.
Freeplus parts
Everything is on GitHub: BOM with sourcing notes, CAD files, firmware and build docs.
- About €26 in parts with the board bought from DigiKey Italy (Oct 2026); roughly €20–60 depending on where you buy the Seeed XIAO board, which is most of the cost. See the BOM ↗
- A 3D printer, a soldering iron and some care with LiPo batteries
- Free for personal and non-commercial use
€19case only
Built your own sensor? This is the case designed for it: a zip case designed for the ODKI-VBT sensor, laser-engraved, with magnets inside that keep the sensor in place. Sensor not included.
- Tracked EU shipping: €9 Italy, €12 rest of the EU
- Max 3 cases per order
- Voluntary 14-day return: case price refunded, shipping not refunded
- Sold by Lodovico Cortelazzo, a private individual in Italy
Orders open early November; request now and you'll get the total by email when they do.Orders are open: you'll get the total by email within 24 hours. Founders batch terms · Photos, details and fit
€129case included
Assembled and tested by hand by the person who wrote the firmware.
- No payment now, no obligation
- You're among the first contacted when the first units are ready
- You hear from me before any payment is asked, and can cancel by replying to that email
- Pre-orders decide how many units the first batch has
First assembled units: once CE conformity and a registered business are in place. Pre-orders hear the date before any payment.
For reference: dedicated VBT devices sold in the EU typically cost €329–587 (list prices checked on 24 October 2026).
Sensor shown for scale: not included with the case on its own.
Companion app
A free app that talks to your sensor.
Live trace, peak and mean per rep, a force–velocity test with estimated 1RM, over-the-air updates. Free to pair and track; paid analysis tools may come later, and the sensor never needs them.
Android: APK on GitHub · Google Play test coming soon · iOS next
Who's behind it
Built by one person, in the open.
I'm Lodovico Cortelazzo. I sprinted the 200 m for the Italian national athletics team, studied sports science and engineering, and now build sport-tech prototypes. I wanted a bar-speed number I could check, so I built one and published how it works.
ODKI is not a company yet: for now it's me, as a private individual.
What you can check today
Source is public
Firmware, hardware files and the protocol are on GitHub ↗.
Pre-order costs nothing
No payment for the sensor. A case is paid only after you confirm the emailed total.
A named person
Lodovico Cortelazzo, background here.
Roadmap
3 done · 4 in progress · 2 planned, all stages.
Order · pre-order · ask
Order a case, reserve a sensor, or just ask.
Payment, shipping, returns, seller
Founders case, €19
- When
- Orders open early November, until 30 November 2026 or until sold out. Requests are handled in order of arrival.Open until 30 November 2026 or until sold out.
- Payment
- PayPal (Goods & Services) only, after you confirm the emailed total.
- Shipping
- Tracked, EU only, per order: €9 Italy, €12 rest of the EU. Ships within 7 days of payment.
- Quantity
- Max 3 cases per order.
- Returns
- Voluntary, within 14 days of delivery: case price refunded; outbound and return shipping are not.
- Sold by
- Lodovico Cortelazzo, a private individual in Italy. ODKI is not yet a registered business (no VAT number). One-off test batch.
Sensor + case, €129
A free pre-order: no payment now, no obligation. You'll hear from me before any payment is asked, and can cancel by replying to that email.
No online checkout and no account. Questions or an existing order: contact@odki.tech
How accurate is it?
We don't know yet, and we won't publish a number until we do. ODKI-VBT is being compared with a linear encoder; the method is public and the raw data will be too (see validation).
What we can say is how it works: velocity is never force-reset, drift is corrected retroactively, and every rep gets a signal-consistency score, a self-check rather than an accuracy estimate. All of it is documented.
What can I buy today, and how do I pay?
The Founders case (€19): a one-off batch of 40, orders open early November,orders open now, until 30 November 2026 or until sold out. Send the form and we email you the total and a PayPal (Goods & Services) request; nothing is charged before that. Shipping, quantities and returns: terms of sale.
The sensor + case (€129) is a free pre-order: no payment is taken, and you'll hear from me before any payment is asked.
Who am I buying from?
From Lodovico Cortelazzo, a private individual in Italy. ODKI is not yet a registered business (no VAT number), which is why the Founders batch is a one-off test batch. Contact: contact@odki.tech.
Does the case fit a sensor I built myself?
It's made for the reference enclosure in the repo's Hardware/CAD ↗. Built a different enclosure? Email its dimensions to contact@odki.tech before ordering and we'll tell you honestly. More on fit.
Can I return the case?
Yes, a voluntary return within 14 days of delivery, no reason needed: write to contact@odki.tech first. We refund the case price; outbound and return shipping are not refunded. A damaged or defective case is replaced or fully refunded at our cost (terms).
Can I use it with another app?
Yes. The Bluetooth protocol is CC0 (public domain), separate from the firmware and hardware licences: anyone can build against it without asking. The ODKI app is one option, not a requirement.
Is the app free?
Pairing, live data and tracking are free. Paid analysis tools may come later; the sensor never needs them.
Is my training data stored in an ODKI cloud?
No. The sensor streams over Bluetooth to whichever app you use, and the ODKI app stores sessions on your phone. No account needed.
If I build my own, can I sell it? What do the licences require?
No, you can't sell it. Hardware is CC BY-NC-SA 4.0: non-commercial use and modification, with attribution, and modified files shared under the same licence. Firmware is GPL-3.0 + Commons Clause: non-commercial use, modification and redistribution; any copy you share keeps the same licence and can't be sold. Selling assembled units is reserved to ODKI. Neither licence is OSI-approved open source: that's what "source-available" means here. The Bluetooth protocol alone is CC0, with no conditions. Full text: LICENSE-HARDWARE and LICENSE-FIRMWARE in the repo.
How does it attach to the bar?
Three neodymium magnets in the sensor's shell (about 2 kg pull each) hold it on a steel barbell: no strap, no clip. Painted or coated steel stays magnetic, though a thick coating may lower the holding force; on aluminium or other non-steel bars it won't hold.
What's the battery like?
A 400 mAh Li-Po, charged over USB-C. The battery percentage is reported live over Bluetooth and re-anchored at every full charge. We haven't measured battery life yet; we'll publish it before any sensor is sold.
Can the firmware be updated?
Yes, over Bluetooth. The app checks the firmware version and offers updates, no cable needed.
Is it a medical device?
No. ODKI-VBT is a training and sports-performance tool. It is not a medical device and is not intended for diagnosis, treatment, rehabilitation or injury prevention.