Max — Hourglass LED Clock

max.dev/projects/hourglass-led-clock

hourglass-led-clock in progress

A sand simulation hourglass clock — old concept, new hardware.
Custom PCB · STM32U073 · dual LED matrices · battery powered.

STM32U073 KiCad 9 IS31FL3733B TPS61022 BQ24232 LP5912 USB CDC I2C IMU wake JLCPCB

~$ cat overview.md

The hourglass clock reimagines a centuries-old timekeeping metaphor using a sand particle simulation running on a microcontroller. Two circular LED matrices sit at the top and bottom of an hourglass form — sand pixels fall from the upper matrix to the lower one, and the rate of fall tracks real time.

The whole thing runs on a single custom PCB — my first PCB design — built around the STM32U073 for its low power profile and native USB support. It charges over USB-C, runs for hours on battery, and wakes from sleep when you pick it up.

~$ cat challenges.md

display driver selection

Finding a driver IC that could handle a full circular LED matrix without excessive external components was harder than expected. After evaluating several options, the IS31FL3733B emerged as the right fit — it supports up to 144 LEDs per IC over I2C, handles PWM dimming in hardware, and two of them can be chained to drive both matrices independently.

power architecture

              Running LEDs, a microcontroller, and an IMU from a single LiPo cell required careful power sequencing. The solution uses a TPS61022 boost converter to supply the LED matrices, an LP5912 LDO for clean MCU power, and a BQ24232 for USB-C battery charging — each stage isolated so the LEDs can't pull noise into the MCU rail.
            

learning PCB design from scratch

This is my first custom PCB. Every decision — layer stackup, component placement, trace width, decoupling strategy — required research and iteration. Working in KiCad 9 and targeting JLCPCB for fabrication added real constraints that pushed the design toward something manufacturable rather than just functional on paper.

~$ ls ./images

+ add schematic screenshot

schematic / KiCad

+ add PCB layout render

PCB layout / 3D render

+ add sand sim demo

firmware / simulation

+ add assembled board

assembled hardware

~$ cat build-status.log

sand simulation firmware complete

schematic complete

PCB layout — in progress

fabrication & assembly

hardware bring-up & testing

→ github ← back_to_projects