Ember Mk.II — Flexible LED Watch

A cyberpunk-inspired wristwatch built on a custom flexible PCB, wrapping high-density alphanumeric LED displays around the wrist in an infinite loop. Displays animated effects and real-time clock output.

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What is it?

Ember Mk.II is a handmade wristwatch built around a custom flexible PCB that wraps 14 Broadcom HCMS-2901 alphanumeric LED displays around the wrist.

How to use it

Press the center button to wake the display and trigger the boot animation. Use the up and down buttons to cycle through menus: time display, mini time, brightness, current, timezone offset, and wave mode. The watch charges and programs over the same 4-pin header used as the wrist. To set the time, send a Unix epoch timestamp over serial.

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PCB Layout

Top Layer	Bottom Layer	All Layers

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Schematic

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Bill of Materials

Ref	Component	Value / Part	Package	Qty	Datasheet / Source
U5–U18	Alphanumeric LED Display	HCMS-3901	Custom SMD (bent TH pins)	14	Broadcom Datasheet · Mouser
U1	Microcontroller	STM32U083KCUx	QFN-32	1	ST Datasheet · Mouser
U2	LDO Regulator	LP2980 (3.3V fixed)	SOT-23-5	1	TI Datasheet · Mouser
U3	LiPo Charger	STC4054	SOT-23-5	1	Datasheet · LCSC
J1	Wrist clasp socket	4-pin SMD socket	custom:SMD_2.54_1x04_LONG	1	Standard 2.54mm pin header
J2	Wrist clasp pin	4-pin SMD pin	custom:SMD_2.54_1x04_SHORT	1	Standard 2.54mm pin header
SW1	Boot mode button	BOOT0	PTS636	1	C&K PTS636 Datasheet · Mouser
SW2	Up button	UP	PTS636	1	C&K PTS636 Datasheet
SW3	Center button	CENTER	PTS636	1	C&K PTS636 Datasheet
SW4	Down button	DOWN	PTS636	1	C&K PTS636 Datasheet
R1, R2	Resistor	TBD	0402	2	Standard 0402 SMD resistor
TP1	Test point	SWDIO	D1.5mm pad	1	SWD debug interface
TP2	Test point	SWCLK	D1.5mm pad	1	SWD debug interface
TP3	Test point	BATT	D1.5mm pad	1	Battery voltage monitor
TP4	Test point	GND	D1.5mm pad	1	Ground reference

⚠️ Display availability note: The HCMS-2901 is a discontinued Broadcom part. Stock availability varies — check Mouser, DigiKey, and secondary markets. Expect ~$25–35 per unit. The HCMS-3901 is a pin-compatible alternative.

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Firmware & Dependencies

Display control is based on a modified version of the HCMS39xx Arduino library:

https://github.com/Andy4495/HCMS39xx

Modifications include:

• printDirectBufferOverlay() — bitwise OR compositing for animation over UI
• printDirectBufferXOR() — XOR compositing for blinking effects
• refreshDisplay() — decoupled buffer writes from display refresh for frame timing

To compile:

• Install STM32Duino (Arduino core for STM32): https://github.com/stm32duino/Arduino_Core_STM32
• Install STM32RTC library: https://github.com/stm32duino/STM32RTC
• Target board: STM32U08 series (Generic STM32U0)

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Resources

Resource	Link
STM32U083 Reference Manual	ST.com
HCMS-2901 Application Brief	Broadcom
STM32Duino Core	GitHub
STM32RTC Library	GitHub
HCMS39xx Arduino Library (base)	GitHub
KiCad EDA	kicad.org

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