The smartwatch category has a battery problem it can’t seem to shake. Despite years of incremental improvements, most wearables still need to be charged every day or two, which is exactly the opposite of what a watch is supposed to be. A watch is supposed to be on your wrist and working, not sitting on a charging pad because you forgot to plug it in before bed.
The LightInk is an attempt to solve that problem by going back to a design philosophy that worked decades ago: solar. The concept mimics the 90s solar digital watches that ran more or less indefinitely, but brings it into the present with an E-Ink display, an ESP32 microcontroller, WiFi, Bluetooth, LoRa radio, and a custom power management system built from scratch over several years.
Designer:
The project started in 2019 with a simple goal: build a solar-powered watch that could send LoRa packets to a receiver at home. After experimenting with early hardware and contributing display optimizations to the open-source Watchy project, the creator hit the limits of what off-the-shelf hardware could manage and built a custom PCB around a TPS63900 buck-boost converter, running the watch at 2.7V.
The biggest technical hurdle turned out to be the microcontroller itself. The ESP32 takes 28ms to boot, consuming around 1mA of current in the process, and that cycle was responsible for about 60% of the watch’s total power draw without contributing anything to the actual display update. The solution was to skip normal boot entirely and run code directly from the ESP32’s RTC memory via a wake stub.
That required reimplementing SPI communication from scratch within the RTC memory constraints, since no code outside that space can run during the stub phase. The payoff was significant: the entire boot, data send, and display update sequence now completes in under 1ms. Once the display is refreshed, the ESP32 immediately returns to deep sleep, saving an additional 1mA that would otherwise be consumed during light sleep.
The result is a watch that runs for six to 10 months on a 100mAh battery, which is already an unusual number for a device this capable. Add the solar panel, similar in type to the kind found on pocket calculators, and the power equation starts to tilt toward indefinite. One hardware revision ran for nine months on battery alone before being retired for a newer build.
The 1.54-inch E-Ink display helps keep those numbers achievable. Electrophoretic displays only draw power when changing states and hold their image indefinitely without any power at all, which makes them an obvious fit for a watch face that updates once per minute rather than 60 times per second. Touch controls via the ESP32’s built-in capacitive touch capability handle navigation, making physical buttons unnecessary and allowing for a more compact case.
The watch supports WiFi, Bluetooth, and LoRa via a Wio-SX1262 radio module, and GPS can be added as an optional component, though the creator notes it wasn’t a particularly good idea given the space and power it consumes. The case is 3D printed in two pieces and accepts any standard 22mm wristband. Everything, including the firmware, PCB schematics, and case files, is open source and available on GitHub.
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