I wanted to find a way to add a voltage display module to one of my C64s, for diagnostic purposes and mostly fun. I came across an interesting article describing how to combine both the diagnostic cartridge and the dead test cartridge and have them switchable, and was halfway through sketching the thing out when I realized it was a work-document for an existing one designed by Sven Peterson. I could then just make his, but I wanted to build one of my own and had a lot of LEDs laying around doing practically nothing - so it became a BlinkenLight project instead.

I'm not sure how much practical use the LED outputs will serve as most signals will be switching faster than is strictly observable. 1 Mhz might be excruciatingly slow by todays standards, but it is still a million signal changes per second on an otherwise functional machine. It would however be able to show lines where a signal is permanently stuck. Taking inspiration from traditional cartridge expanders I've added a third switch that can be used to disconnect cartridge detection signals, allowing the machine to boot normally with the cartridge still inserted.

Check out the github-repository for the BOM as well as build instructions. Note that there is also an additional faceplate that goes with this project, it hides most of the electronics and adds some much needed readability to the readouts.