Charliewatch Simulates an Analog Watch Face with 72 LEDs

While better known for frequently taking things apart, in this case, Trammell Hudson put together a digital "analog" watch called Charliewatch. The eye-catching, yet simple, design only needs a CC430 MCU, CR2032 battery, and a bunch of surface mount LEDs.

Charliewatch's face contains 72 LEDs! There are 60 for minutes and 12 for hours. Thanks to a technique called Charlieplexing, the MCU only needs 18 IO pins to drive all of the SMD LEDs. This method is similar to multiplexing. The difference is that when a matrix contains only LEDs, it takes far fewer IO pins than traditional multiplexing.

At the heart of the watch is a TI CC430 microcontroller. The CC430 is an interesting choice. It is a system-on-chip (SoC) with an MSP430 CPU and an RF transceiver. Charliewatch does not make use of the CC430F5137IRGZR's RF capabilities. (On the most recent version of the PCB, v0.2, there are no RF components to support the CC430's transceiver.)

Charliewatch targets the size of a smartwatch. The PCB fits inside of a 1-inch square with a finished footprint of around 36-38mm. The example case is about 40mm.

The GitHub repository contains the code, schematics, and an OpenSCAD script for the case shown in the pictures. There are multiple revisions of the schematic and PCB provided as EAGLE files. If you are a KiCad user, its importer will open the design, although the PCB lacks the copper pours.

For more information, check out Charliewatch on Hudson's website.