Just a quick post… Yesterday I wrapped up the schematic and printed circuit board (PCB) design for the GPS clock, and built it. One software change was needed — in the course of laying out the pushbuttons, it worked better use to them in a different order (e.g., up was formerly button 1, it became #4). Thankfully that soft of change is trivially easy to make in software. Of course I built the board with a 6-pin AVR programming header to enable software changes.
This was my best experience yet with toner transfer — I cleaned the copper really well first, then lightly etched with white vinegar, scrubbed with 0000 steel wool, and finally wiped with acetone. Just like in painting, surface preparation is apparently where success is determined.
I like the clock with its guts showing, though maybe I’ll use an enclosure on the next one.