I'm proud to announce OpenStint, an open-source, RC3-compatible laptiming decoder project. Instead of custom hardware builds, it relies on inexpensive, ready-made software-defined radios. The primairly audience are small clubs and friendly gatherings, who either like thinkering or have no budget for a ready-made solution. It is also useful for tracks where RC3 compatibility is required, but compatible decoders are not available any more.

PROJECT URL: github.com /zsellera/openstint

The project is accompanied by a reference design of a transponder, and a low-noise balun & preamplifier side-project, custom-designed for RC timing loops.

HIGHLIGHTS

• Off the shelf components only, no soldering is required. The heart of the decoder is a HackRF One, a software defined radio, available from $90 at opensourcesdrlab.com.
• Supports RC3 protocol, with error-check and a makeshift error correction (little-to-no "ghost"/"shadow" transponder ids).
• Defines (and decodes) an open transponder protocol. Known, easy-to-understand algorithms allowing 3 dB signal-to-noise ratio improvements (see docs/).
• Runs on a Raspberry Pi Model 3 B+ (sub-$40 single board computer). A sub-$200 permanent installation is possible.

To watch it in action, here is a quick video: youtube.com /watch?v=YDW0eA1Szk4

To start using it, the necessary steps are detailed in the project's README. In short, you'll need:

• HackRF One; RTL-SDR support is planned, but not available now.
• A 1:9 or 1:8 HF balun (search for "noelec 1:9 balun" it's a $3 stuff) or a magnetic field probe.
• Some 50-ohm coaxial cable (RG58 and RG316 are the inexpensive options)
• An optional (but highly recommended) termination resistor: 330 ohm for surface installations and 470 ohm for above-the-track installations (see TDR measurements)

Note however, the project is in-the-making. See project roadmap in the aforementioned README file.

PICKUP ANTENNA
The detector antenna uses a similar design as other commercial solutions: it is a parallel wire transmission line, with a balun on one end, and a 330/470 ohm termination on the other end. The wires should be ideally 2 mm^2, and separated by 25 cm from each other. Use the 330 ohm termination for in-the-ground installations, and the 470 ohm termination for overhead wires. The inexpensive NoElec 1:9 HF balun gives a very good impedance match for the overhead installations, and is also usable for in-the-ground setups.

Note: I use the term "antenna" and not "loop". From a geometrical standpoint, it is a loop for sure. From electrical perspective, this is a parallel-wire transmission line, an unshielded arrangement to pick up external disturbances. There is no need for "resonance matching" or other similar magic.

TRANSPONDERS
While the project can decode RC3-compatible transponders, the preferred protocol is the OpenStint transponder protocol. The protocol is well documented, it makes sense (no deliberate obfuscation) and has pretty good SNR characteristics. A reference implementation is available (schematics, pcb and firmware). At club level, one can manufacture 40 pcs of these transponders for $130 (a cost less than a single RC4 hybrid).

The reference design is available at:
github.com /zsellera/openstint-transponder/

The reference design produce comparable signal levels to the commercial offerings when running from 7.2 V (servo port of the receiver). It requires 4.5 V minimum to function properly, and testing was done up to 8.5 V (2s). The output level depends on the input voltage.
Note: this is a "v2" design shared, I just submitted an order at JLCPCB for it. I'll update this thread when I could properly test them.

PREAMPLIFIER
The project comes with an optional, low noise preamp + filter, that does the differential to single-ended conversion as well. It fits into a Hammond 1590L die-cast aluminium case. It is useful at overhead antenna installations and in noisy environments. The design offers a good +13 dB power gain. The onboard filters get rid of out-of-band interferring radio signals, which may get downconverted to baseband due to non-idealities in the inexpensive radio receivers.

Project url:
github.com /zsellera/openstint-preamp

WHAT'S NEXT?
On the short run, I'd like to focus on:

• lapcounter software: application for practice sessions, maybe clubraces later on.
• STL-SDR v4 support: the cost of a 3rd-party HackRF One is ca. ~$90, while the cost of an original RTL-SDR is ~$50. On the other hand, RTL-SDR implementation is likely consists of a fractional resampler, which is resource-intensive (might need Raspberry Pi 4 or even 5).
• sector timing: while this is primarily a lapcounter software feature, the clock-syncing required for it is a "first-class citizen" of the OpenStint project. See the "timesync messages" in the transponder protocol documentation.

I've spent about 2 months of a sabbatical on this project. While I think it was the best possible way to spend this time, I'd like to return to the world of 9-to-5 soon. As such, there are no promises on future features or project timelines.

HOW CAN YOU HELP OR CONTRIBUTE?
First off, this project converts a hardware problem into a software one, which is usually more likely to get solved.

• This project severely needs integration with laptiming software. If you develop one, and you're interested in an integration, check out the decoder protocol documentation.
• The project contain some basic integrations in the integrations folder, including a very basic laptiming software (for educational purposes). I'd like to work on a P3 bridge soon. If you have any documentation on the P3 protocol, please share it with me.
• While reading the specs of the reference transponder, one might notice the gap between this and the RC4 transponder. Someone shared an X-ray image of an RC4-hybrid on this forum, and a "V7" text is visible on the copper layer. The design I shared is "V2" as of now. On the other hand, I unfortunately out of ideas on how to push it any further.
• If you have developed your own transponder, let's talk. A reference implementation of the transponder protocol is available in github, and I can help making sense of it.
• Test it out, preferably at your local track. I'll do the same. Initial tests are promising btw.

Also, if you like this project, please star them on Github.