I finally sat down to seriously take a look at why my shift register boards weren’t working. It took me all of 2 minutes with my multimeter to figure out what the problem was. It’s actually embarrassing: The Data In pad on the board isn’t connected to anything. I checked the schematic again and sure enough there’s the lone pad in the lower left corner. Somehow, when combining the Attiny85 programmer/breakout board with this one to fit on the 5x5cm board, the trace must have been left out as the original design file has it.
The fix is really simple once headers are in place. I just need to attach a jumper wire from the Data In header to pin 14 of the left shift register.
That was the only problem with it! I set up a test and it worked as it should. Here’s a video of my sample program:
I ordered some more shift registers, as well as some sockets and diodes. I’m prepared to assemble a mass of boards.