I have dusted this project off after a bit of a hiatus and a move to a new place. I have now scanned the manual for the HHCH and uploaded it to the project. I wasted a lot of time today trying to find a spreadsheet mapping the control head ultimately to my breadboard. In the end it turned out that I had never recorded that information off my white board because I knew it was in the manual. Once I remembered I had the manual, I decided it ought to be digitized so it could be added to my collection of project files to reduce the chance of losing what was my only copy.

I couldn't make heads or tails of what I was doing to switch the second 7805 on and off with a single transistor. My attempts to make it work failed. In the end I settled for switching the output. Both 7805s will draw from the 12v battery lead all the time. I'll have to measure the current draw, but I imagine it's fairly low when the HHCH is off.

I still need to make sense of the rest of the project. It's really tempting to clear the breadboard and start over. First I'll see if I can't make sense of what I've got.