OpenTrailPaper: open-source firmware turns an e-paper board into a bike computer

OpenTrailPaper is an open-source firmware project that turns a 4.7-inch e-paper development board into a bicycle computer. It runs on the LilyGO T5S3 4.7" E-Paper PRO, shows ride data and offline maps, follows GPX routes, records FIT files and pairs with Bluetooth sensors. Riders buy and protect the board themselves.

What the device does

On the bike, the head unit tracks GPS speed and distance on a live map and records each ride to a .fit file on an SD card. It gives turn-by-turn prompts along a loaded route and can run structured workouts from .erg or .mrc files. Heart-rate and power sensors pair over Bluetooth 5. Because maps and routes live on the card, the unit keeps working without a phone or a data signal once they are loaded, and there is no account or subscription.

The phone app is optional

iOS and Android companion apps handle setup and transfer: planning a route and sending its GPX file to the device, building offline maps for a new area, creating workouts, arranging device screens, copying recorded rides back for review, changing settings such as FTP, timezone, units and backlight level, and installing firmware updates over Bluetooth. The iOS app is free and requires iOS 17 or later; the Android version is a beta APK for Android 8 and above, installed by sideloading. During a ride, the phone can stay at home.

Hardware and honest trade-offs

The project currently targets one board: an ESP32-S3 with 16 MB of flash, 8 MB of PSRAM, a 960×540 e-paper panel, GPS, capacitive touch, a front light, a battery and USB-C. E-paper stays readable in direct sunlight, and firmware is installed from a desktop Chromium browser over USB. The site lists the compromises plainly: there is no barometric altimeter, so total ascent is estimated from elevation baked into the map tiles; the GPS is single-band and would hold a fix better with a multi-band module; without a magnetometer the map only orients from direction of travel; a 1,500 mAh build ran about 7.4 usable hours with the front light off, against a measured baseline of roughly eight hours; hardware buttons are weak, leaving most interaction to a touch panel that is fiddly with winter gloves or in the rain; and the board ships bare, with no seal or IP rating, so it needs a case before wet rides.

Open to contributions

The authors describe OpenTrailPaper as a development project rather than a finished, weatherproof product, and they are asking for help: a better target board would add a barometric altimeter, multi-band GPS, a magnetometer, a larger battery, usable buttons and a weatherproof case. Suggestions arrive as issues in the source repository, where ports to other boards are also welcome. Two sponsors, RideCall and Hummingbird Fuels, support the work.