About word clocks…

A bit over a decade ago I built a word clock – one of my first microcontroller projects. The project was fairly expensive: It uses a large stainless steel front panel (ca. 39cm x 39cm, laser cut) and a similar sized PCB. Over the years I had to resolder one or two of the RGB LEDs and replace a failed one, but other than that it’s still working as intended.

The new, small work clock in front of one of my first microcontroller projects from more than a decade ago

Then one or two months ago, I saw a post on hackaday about a tiny word clock – using a pretty clever and simple design: member sjm4306 just used a PCB with black solder mask as a front panel. This is feasible because the PCB’s material (FR-4) is translucent enough to let a significant amount of light pass through; however light can be selectively blocked by just leaving the copper in place. At the same time, the FR-4 material works as a diffusor in front of the LEDs that illuminate the characters. And the solder mask determines the color of the panel. Nice!

For my version I replicated the 11×11 character matrix design of the large word clock mentioned before. One advantage of the 11×11 matrix (compared to different designs with less rows and/or columns) is that I can actually use it as a matrix display with 2×2 characters of 5×5 pixels each. This proves really handy for the clock settings menu. Since I was able to re-use some of the code I wrote a decade ago for the mentioned large clock, the new clock has alternate display modes too: A numeric clock and a binary clock display.

A STM32G0 series microcontroller with a dedicated VBAT pin for the RTC peripheral multiplexes the LEDs. Since the microcontroller sources the current of up to 11 LEDs at a time (column) I had to limit the current to something like 5 mA. The LED’s brightness is kind of ok, but certainly not great – despite the fact that I use fairly efficient white LEDs. One potential reason: I forgot to select a lower PCB thickness and so I got a standard 1.6 mm panel. I don’t intend to, but if I ever did a revision B I probably would add high side switches (i. e. P-channel MOSFETs) to allow for higher LED currents. Also worth mentioning: The FR-4 will inevitably give the light a yellow-ish tint. Choosing a white LED with a high color temperature (“cool white”) might or might not help here a bit, haven’t tried.

The controller board with a STM32G0 series microcontroller. I didn’t label the switches correctly. Also, I forgot to reserve the LSE clock output pin.

I calibrated the RTC with my frequency couter which worked really well using the LSE clock output pin. (Don’t forget to reserve this pin for this purpose like I did.)

Initially I struggled with two of the general purpose inputs used for the switches. Allthough I activated the pull ups, the voltage level was fluctuating and often closer to zero than VDD. At first I suspected that it might have something todo with the multiplexing, but no: On both suspect pins the microcontroller has pull-downs for USB C PD that are active by default (reading the documentation helps). Deactivating those explicitly solved the issue. STM itself seems to be confused about it too, stating on a few occasions in the HAL library/Device configuration tool that you can deactivate the “pull ups” instead of “pull downs”…

Model of the 3D printed “enclosure”, guiding the light of the LEDs to minimize light bleed to adjacent characters





WordPress Cookie Notice by Real Cookie Banner