Boolean logic and 7400-series ICs
In order to drive the relay modules, a set of control signals is needed. Since I only have 18/5 cabling at most, that means that I'll only have 3+VCC+ground to work with, but there are five relay modules plus a reset line. Boolean logic to the rescue!
My thinking was using binary number values from 0-4 to select which relay to enable, while I'd have a 7 (0b111) indicate a reset signal. After creating the truth table on paper, I converted it into AND and NOR gates and simulated it in Qucs-S.
Then I made footprints for the 74HC00A-series ICs that contain the gates I'd be using, and lastly created a schematic showing the routing for all of the lines I'd need in KiCad.
With that, I had what I needed to actually create the board on perf board. I also made a test fixture using buttons and LEDs to verify that it was correct.
The only bug I ran into was where the Relay4 line was high whenever I held all buttons down, which should only trigger Reset. Turns out I only forgot to connect a couple of traces. With that fixed, it worked 100%!