@Ray; sorry I didn’t respond sooner. I checked back a couple times and didn’t see your post, and I had the “Notify me of follow-up…” checked but never got notification. In any case… yes, despite the fact that I said “SCRs” I meant a shift register and triacs/relays/SSRs. If I can drive a shift register at the end of a 50′ wire run, it should do what I need. I want to keep it small because I don’t ever anticipate putting even 8 valves in a box with the current layout of my irrigation system, but I’ve got a place where there used to be one valve and I’ve grown it to 3.
I thought I posted another message to this thread, but it’s not here so maybe I got sidetracked and didn’t press “send”. (DOH!) One solution I found since the original post would be to convert the signal I’m sending down the wire to DC, send one polarity down for one valve and the opposite for another valve, and have diodes in series with the valves to get them to respond to the proper voltage. This could be done with a 24VAC relay that would be driven by the OSPi and a rectifier circuit to convert the 24VAC to probably about 12VDC (I’m not sure about the AC-DC voltage conversion to drive the valves). This wouldn’t allow much expansion, but it should work to add a single valve.
Maybe I’ll throw together a mini version of the expansion board, give it a try, and post my results here… as time permits. If it works okay I’ve got a couple more valve boxes that could use an extra valve to add a zone, but none of them would have more than 4 valves so a full expansion board wouldn’t be practical.