OpenTrailPaper Turns E-Ink Boards Into DIY Bike Computers
A new open-source firmware project lets you build a GPS bike computer with offline maps and Bluetooth sensor support.
Tools · Source: Hacker News
What happened
OpenTrailPaper just launched as an open-source firmware for the LilyGO T5S3 e-paper development board. It turns bare hardware into a fully functional bike computer. You buy the board yourself. You flash it via a web browser. You build your own case to protect it. The system handles GPS tracking, offline maps, and Bluetooth sensor pairing. It supports heart rate monitors, power meters, and cadence sensors. It even runs structured workouts from standard file formats.
The software operates independently during a ride. It records standard FIT files to an SD card. It displays turn-by-turn navigation on a 4.7-inch sunlight-readable screen. You do not need a phone connection while pedaling. There are no accounts or subscriptions required. An iOS companion app handles route planning, offline map generation, and firmware updates over Bluetooth. Android support is currently in closed beta.
The project is strictly for tinkerers right now. The target hardware has clear tradeoffs. It lacks waterproofing, a barometric altimeter, and a compass. Total ascent is estimated from map data rather than measured directly. The basic GPS module struggles under heavy tree cover. Battery life sits around eight hours on a 1500 milliamp-hour cell. The creator is actively looking for developers to port the firmware to better boards with multi-band GPS and weather-sealed cases.
Key facts
- 960x540 — Resolution of the supported e-paper display
- 16 MB — Flash memory on the LilyGO T5S3 board
- 7.4 hours — Usable battery life with a 1500 mAh battery
- 1 Hz — Recording frequency for FIT files
Why it matters
Hardware hacking is getting dramatically more accessible for software engineers. The Web Serial API allows flashing firmware directly from a desktop Chromium browser. You no longer need a complex local toolchain or command-line experience to deploy embedded software. A solo developer built a legitimate alternative to expensive Garmin or Wahoo head units using off-the-shelf ESP32 parts and open-source code. This lowers the barrier to entry for building custom hardware products.
This exposes the massive markup in consumer sports electronics. The core logic of GPS tracking, sensor pairing, and screen rendering is now commoditized. Open-source projects can leverage cheap microcontrollers and standard Bluetooth protocols to match proprietary features. As these community-driven hardware projects mature, they will pressure legacy fitness brands to justify their high prices. We will see more niche, community-driven hardware devices emerge for specific outdoor sports. Builders can now prototype hardware businesses with minimal upfront capital.
For builders
Web Serial API simplifies hardware onboarding
OpenTrailPaper uses a web flasher to install firmware directly from a desktop browser. This removes the friction of installing drivers and command-line tools for end users. Hardware startups should adopt this pattern immediately. It lowers customer support costs and speeds up user adoption for DIY products.
Opportunity to build better target hardware
The current LilyGO board lacks weatherproofing, an altimeter, and multi-band GPS. Hardware engineers can capture this niche by designing a custom board specifically for this open-source firmware. Cyclists will gladly pay for a rugged, ready-to-ride alternative to proprietary head units. The software is already written and waiting for better hardware.
Companion apps handle the heavy lifting
The e-ink device does not waste precious battery rendering map tiles from scratch. The iOS app processes offline maps and routes before transferring them over Bluetooth. Offloading heavy compute to the smartphone is a proven pattern for extending battery life in embedded devices. Builders should keep their edge devices dumb and their mobile apps smart.
My take
I love seeing solo builders attack hardware monopolies. Garmin and Wahoo have been selling overpriced, sluggish devices for years because they owned the ecosystem. OpenTrailPaper proves you can replicate the core value with cheap ESP32 boards and smart software. This is how you disrupt legacy hardware.
Original reporting: Hacker News. This is my rewrite and opinion.