Summer break is here, and with some time to spare I decided to challenge myself with a project that I call “Clocksquared Mini”. It is Clocksquared, but in a tiny wristwatch package. This gives rise to a major challenge, as everything has to be shrunk down approximately ten times from a 300x300x50 mm to an approximately 35x35x7 mm package. Moreover, running everything off a tiny battery whilst maintaining an acceptable battery life also turns out to be quite difficult.
I already have made quite some progress that I would like to share. I recently finished designing the main PCB, which is a 2-layer board. It features a 10×10 Charlieplexed grid of 0402 SMT LEDs (e.g., individually addressable). The back side of this PCB contains an ATMega328P(-AU) microcontroller, a DS3231MZ+ RTC module and a CR12xx battery holder, amongst some other components such as some resistors and a push button. Note that I specifically chose the MZ+ version of the DS3231 RTC because it comes in a smaller package (SOIC-8) than the “normal” one. I had the PCB manufactured and assembled (top side only) and here is the result:
I am very happy with this result (especially the quality of the manufacturing), even though I already came up with a few design improvements that I will implement for the second prototype. I have not yet tested the LED array and how well Charlieplexing will work in my case. I’ll discuss that later, when I assemble the bottom side of this PCB.
For the front of the clock, I was inspired by a YouTube user called sjm4306. I came across his excellent idea to use a thin PCB with only a solder mask pattern with the 10×10 letter grid that makes up the clock. The 0.8 mm thick FR-4 PCB material acts as a diffuser and the solder mask pattern ensures that only the letters are translucent. The result is excellent, with much better resolution than I had expected. Previously, I was worried that I would’ve had to use e.g. micromilling techniques to obtain this front plate; who knew conventional PCB manufacturing techniques would yield a much better, simpler, cheaper and more scalable result?
I am currently designing the housing for this watch, writing the firmware for the microcontroller and obtaining the right tools to assemble the bottom side of the main PCB (I previously used a low-quality iron; that won’t cut it here). Stay tuned for more!
The KiCad files for the front PCB and the electronics PCB can be downloaded here. Make sure to read the readme file because the gerber files for the front PCB cannot be used as-is if you choose to generate them yourself from KiCad!