The LightInk Watch Pairs a Solar Panel with ePaper to Deliver Theoretically Infinite Battery Life

Mononymous telecom engineer Daniel has designed a smartwatch for the energy-conscious — using sleep modes and an ePaper display to deliver a LoRa-connected wearable powered solely via harvested solar light.

"This project started long time ago in 2019, with the idea to build a solar watch that can use LoRa packets to communicate to a receiver at home," Daniel explains of the Lightink. "Back then I started with a Heltec Wireless Stick Lite and an external [ePaper] display. Then I discovered [Sqfmi's] Watchy [ePaper smartwatch] and immediately bought one, [but] it didn't have what I wanted (sleek design, LoRa, and solar), however, I thought it was a very good foundation. Back then, I contributed to Watchy many patches to optimize the display as much as I could, but at some point that was not enough and new hardware was needed."

That search for new hardware led to an entirely new watch, built around an Espressif ESP32-PICO-D4 microcontroller module, a Wio-SX1262 radio for the LoRa connectivity, and a compact 1.54" ePaper display — an electrophoretic panel that requires power only when changing states, and which can be powered down entirely while still retaining the last-shown image. Running at 2.7V, below-spec for the parts but seemingly functional, Daniel got battery life up to two months before hitting a wall.

"The ESP32 takes 28ms to boot, and that uses around 1mA of current," Daniel explains. "About 60% of the total power was being used by the ESP32 just to boot (not to update or display anything). This is when i had an idea, not boot the ESP32 at all, use a wake-up stub to run RTC [Real-Time Clock] code. I implemented code to handle the SPI communication and to update the region of the display controller buffer. With this I could boot+send+update display in less than 1ms."

With the new low-power system in place and a compact solar panel — of the type found on the calculators once-common to everyone's desk — the watch's battery life is, theoretically at least, infinite. "I finally managed to have a watch with battery running [for] nine months," Daniel says, "until I decommissioned it for a new [hardware revision.] Since then I have polished the code, the case, added LoRa (yeah!), even GPS (was a bad idea), and I am still building the project up."

More information is available on Hackaday.io, while source code and hardware design files are available on GitHub under an unspecified license.