Sahko’s Custom-Built Futuristic Watch Has Retro-Tech Charm, Thanks to Flexible PCBs

Sahko recently showed off his latest creation at OpenSauce, a maker gathering. His first project is a custom smartwatch that was built around a flexible circuit board, complete with glowing amber LED dot-matrix displays, making it a retro-tech dream.

This device gets its soul from the HCMS2901 LED displays, tiny amber grids that look like 1970s calculator displays. These displays were originally designed for industrial use and wrap around the wrist in a continuous loop, creating a beautiful band of light. Each display is a small miracle, packing a 5×7 pixel grid into a tiny package. Sahko used 15 of these, but recommends the HCMS3901 for better 3.3V compatibility and availability. These displays are expensive – $30 each, $420 just for the screens. They’re power hungry too, drawing a full amp at max brightness, so the watch defaults to “off” to conserve battery, like the old pulsing LED watches of yesteryear.

Powering this glowing spectacle is a salvaged lithium-polymer battery from a disposable vape, a clever choice that keeps the design slim and light. The battery, paired with an STC4054 charger IC and 3.3V regulator, gives just enough juice to keep the watch ticking. But it’s not without flaws. At full brightness, the battery lasts 30 minutes, so Sahko added buttons to toggle the display and adjust brightness. These buttons are embedded in the band and let you cycle through modes, including a hypnotic waterfall effect that cascades across the displays like digital rain from The Matrix. It’s a showstopper, that turns the watch into a conversation piece.

Instead of a traditional strap, Sahko designed a flexible printed circuit board (PCB) to house the displays, microcontroller and charging circuitry. This flex PCB from JLCPCB wraps around the wrist with stiffener layers to support components that can’t bend without breaking. There’s no traditional clasp – a 4-pin connector serves as both the closure and the charging and programming interface. This dual purpose design is clever but also shows the watch’s fragility. Sahko says USB-C connectors were too rigid and would damage the PCB so he used a salvaged USB cable with a pin header instead. It’s a hacky solution but it works.

At the core of the watch is an STM32U08 microcontroller, a tiny chip with a built-in real-time clock (RTC) that keeps accurate time. Unlike the bigger Raspberry Pi Pico Sahko originally considered, the STM32U08 is power efficient and only needs a power supply to work. USB programmability via the same 4-pin connector used for charging makes updating the watch’s code a breeze. Sahko’s software on GitHub drives the displays with animations and timekeeping. A power supply issue where voltage dips caused some displays to reset was a problem during development. Adding capacitors to the logic lines stabilized the system, but Sahko had to add them after the PCB was made, a testament to the experimental nature of the project.

Building the watch was a time crunch. With OpenSauce looming Sahko had tight deadlines, designing the PCB and coding the animations in a mad dash. The flex PCB arrived just as he was leaving for the airport, so he had to assemble it in Denver at a friend’s house. Soldering was another challenge—the HCMS2901 displays can’t handle high temperatures and standard hot air soldering would damage them. Sahko switched to low temperature solder paste and a hot plate, but the brittle solder would snap off during use. He repaired the watch at least 10 times over the OpenSauce weekend and eventually rebuilt it on a new flex PCB. If you want to build your own Sahko’s GitHub repositories are a good starting point, but sourcing the displays is the hard part.
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