This is a recapture of the Tom Arnold schematic hosted by ReactiveMicro. (A copy is included within for reference.) If I had to guess, it was produced with EAGLE, which was a reasonably popular EDA package at the hobbyist/maker level at the time. This time, I've rearranged the schematic in KiCad, which is fully open-source and likely to remain so for the foreseeable future, while equaling or exceeding EAGLE in its capabilities (without the subscription nonsense that Autodesk has bolted onto EAGLE since its acquisition).

With the schematic, I've tried to keep signals mostly moving from left to right. I've added a few bypass capacitors that weren't in the original schematic, but show up in photos of the assembled cards. I've applied some of the fixes to the design issues noted in the ReactiveMicro wiki.

As of this writing, the PCB layout is in progress. The major components are where they were in the original, but some of the others are likely to end up in new locations.