3/4/2026 11 AM - Designed a flexible PCB wrist display

I've been thinking about making a wearable device that has several OLED screens around my wrist, like a watch.
The idea is to make a flexible printed circuit board (PCB) that has a microcontroller, charging circuitry, and display function all in one small package. To make sure it fits right, I need to measure my wrist accurately. I plan to do this by cutting a strip of paper to get the right size. Early estimates say that the wristband has enough room for about 15 individual displays.
I'm thinking about getting rid of the Raspberry Pi Pico when it comes to hardware. It is a powerful choice, but it is also quite big and needs external flash memory and a crystal oscillator, which makes it less useful for me. Also, it uses more power than I would like for this project. I'm thinking about the STM32U083KCU6 microcontroller instead. It's more efficient because it only needs a power supply, which makes the design cleaner and more streamlined.

3/4/2026 12:00 PM - Got the matrix waterfall animation working

I began the process of writing firmware for the STM32 microcontroller. When the watch is powered on, it will show a Matrix-style waterfall spinny effect, which will then seamlessly transition into displaying the current time. I have successfully developed a working prototype that is currently running on a breadboard setup. This allows me to prepare the code thoroughly before the printed circuit board (PCB) even arrives, ensuring a smooth transition to the final hardware.
During the development, I encountered an unusual bug that surfaced after a few seconds of operation, where some of the displays would unexpectedly cut out. After some investigation, I traced the issue back to power supply spikes. It turns out that every time a display resets, all the pixels light up simultaneously, causing a significant voltage dip that resets the logic of the system. To resolve this issue, I implemented bypass capacitors on the logic lines. After making this adjustment, I used an oscilloscope to confirm that while the spikes are still present, they are now much smaller in magnitude. As a result, the displays remain operational and stable, which is a significant improvement for the overall functionality of the watch.

3/4/2026 12:08 PM - Firmware Complete

Successfully put the full firmware on a prototype watch.
This new device has seven different menu states that you can move between using the up and down buttons. The available states are time display, mini time, brightness control, current control, timezone offset, and wave mode. These give users a flexible experience that can be customized to their needs. The internal clock of the STM32 microcontroller powers the Real-Time Clock (RTC), which keeps time accurately. Users can set the time using Unix epoch time through a serial connection that is easily sent over the same USB cable that charges the device.
The display architecture is made up of three HCMS39xx objects, each of which can hold 16, 20, or 20 characters. This setup makes it possible to get a lot of visual output. A bitwise OR overlay is used to put the animation buffer on top of the current menu rendering for each frame. This smart design makes sure that animations work separately from the user interface, which makes for a smooth and interesting experience for the user. Overall, making this firmware is a big step forward for the project, and it will allow for more improvements and features in future versions.

3/4/2026 12:21 PM - Finalized PCB design

The design of the PCB is finished. The last layout is a single, continuous flex board.
with the STM32U083 microcontroller, the STC4054 charging IC, and the LP2980 LDO
the regulator, all 14 HCMS-2901 displays, three buttons, and the pin header
A strip that wraps around the wrist holds everything together.

Added stiffener layers under all the component pads so nothing breaks off.
when the board bends. The display footprints are made to order for the HCMS-2901.
is through-hole, but I made an SMD footprint by measuring the pin.
pitch and bend the legs flat. All of the bypass caps are put as close as
each display's VLOGIC pin as much as possible to fix the power spike problem
found while testing the firmware.

I chose the STM32U083 because it has built-in support for USB.
The same pin header that holds the watch together also programs it.
and a place to charge it. No need for an outside programmer.

3/6/2026 - 3D modelling or atleast tried to

Sourced the PCB STEP file from KiCad and converted it to STL, then sourced a STEP file for the HCMS-2901 display and converted that to STL as well. Got both into Tinkercad and designed the final assembly from there.