Tinta

Tinta is a battery-powered ePaper sign built around the Raspberry Pi Pico W. A real-time clock wakes it on schedule, it fetches data over Wi-Fi, redraws the screen, and then switches itself off completely in hardware — there is no sleep mode. A push button or an NFC tap wakes it on demand. This approach earned an Honorable Mention in the "Least Power" category of Hackaday's 2026 Green-Powered Challenge. One set of ordinary alkaline cells lasts thousands of refresh cycles, and the ePaper keeps its image while the device is off.

This repository holds everything needed to build one: firmware, PCB design, 3D-printable enclosure, and an optional telemetry service. Formerly published as inki.

Crowd Supply campaign · Hackaday logs · Mastodon · MakerTube · YouTube

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Try it on a bare Pico W

No custom hardware required — the firmware runs on a stock Pico W with the display disabled.

1. Download a prebuilt UF2 from the latest release: seatsurfing · historian · homematic
2. Hold BOOTSEL while plugging in the Pico W, then copy the UF2 to the RPI-RP2 drive.
3. The onboard LED blinks inki in Morse once it is up.
4. Connect to the Wi-Fi access point inki-setup and open http://192.168.4.1.

Release artifacts and the setup access point still carry the old inki name.

Build from source

Building, flashing, use-case selection, and the debug output are documented in firmware/README.md. In short:

cd firmware/c
./build.sh --seatsurfing        # or --historian, --homematic, --weathermap

Requires the Pico SDK 2.1.0 (PICO_SDK_PATH), the ARM GNU toolchain, and CMake 3.12+. The build produces a bootloader, two firmware slots for over-the-air updates, and a default configuration image.

Repository layout

firmware/       Pico W firmware in C — build system, use cases, drivers, web interface
hardware/       KiCad schematic and PCB (circuit/), FreeCAD and STL enclosure (enclosure/)
inki-monitor/   optional Python/Flask service collecting battery and Wi-Fi telemetry
scripts/        helper scripts
images/         photographs and diagrams used in the documentation

Each directory has its own README with the details.

Hardware

The enclosure is modular and 3D-printable, with a dovetail mount: the baseplate is glued or screwed to the wall and the sign clips on, so it lifts straight off for a battery change.

The circuit is a custom PCB carrying the Pico W, a real-time clock, MOSFET power switching, battery monitoring, and an optional NFC front end.

NFC

Boards with the optional ST25DV04KC tag can be reached by a phone while the device is switched off: the phone's field powers the tag, the phone writes a short command into its EEPROM, and the tag pulls the wake line. The Pico W then boots, reads the command over I²C, and acts on it — so a tap can refresh the display or book a desk without the device having been powered at all.

The 16-byte wire format, the wake-source decision tree, and the antenna tuning helper are described in firmware/README.md and firmware/c/README.md. The companion Android app lives in its own repository, wake-electronics/tinta-tap.

Field results

A 4.2-inch unit running the Seatsurfing use case has been logged continuously in an office:

393 days, 5142 wake cycles on one set of three AAA cells
99.5 % day coverage; one outage in 393 days (66.6 h, external cause)
45-minute wake interval, median 2700.0 s over 13 months, no measurable drift

The 4.2-inch version is rated at about 5,000 wake cycles on three AAA cells, the 7.5-inch version at about 10,000 on three AA cells. The unit above met that rating in the field and kept going. Actual runtime follows the configured wake interval: a longer interval means fewer cycles per day and a longer life on the same cells.

License

This project contains both hardware and software components, licensed under separate terms:

• Everything in /hardware is licensed under the CERN Open Hardware License v2 — Strongly Reciprocal (CERN-OHL-S-2.0).
• All original content in /firmware is licensed under the Apache License, Version 2.0. Bundled third-party components under /firmware/c/third_party/ retain their upstream licenses — see firmware/c/third_party/README.md.

See the LICENSE files in the respective directories for the full texts.

Contributing

Issues and pull requests are welcome, particularly new use cases: one directory under firmware/c/ and one build flag, see firmware/README.md.

Contact: c0de@posteo.de