For mounting the solar panel model, I needed to increase the displayboard size. I retained the Stepper mount at the center of the LED ring. The LED ring grew to 16cm diameter, bringing the LED count to 48. Together with the 16-LED Zenith row on the mainboard, the total of 64 dual-color LEDs require 128 IO pins. This is going to max out the number of DIP IO expander IC running on a single I2C bus.
Creating the PCB layout was a real challenge. KiCad currently only supports creating tracks with 90 and 45 degree angles, but the tight spacing in a circle demands angles in between. The workaround in KiCad is to create a track stub, and hand-edit the start/end values in the track parameters. A exercise in diligence completed the job. Component placement with uncommon angles is tricky as well, each LED/resistor pair has been hand-fitted. For the final touch I decided to round-off the dual-layer PCB.
Compared to the hard work in PCB layout, PCB assembly was no big deal. However with mainboard and displayboard designed and build separately one month apart, the question is: would they fit together?