I converted my Hardi sprayer box to take section control from AgOpenGPS (replaced the internal PCB).
I dont know if it is the same with other sprayers, but the Hardi valve bodies work using a small motor with limit switches open/close the valves, so you need to use changeover relays rather than just on/off ones so you reverse the polarity for close vs open and vice versa.
/Edit: The pressure increase/decrease signals are slightly different in that they operate a single motor, with a NC signal to ground on the high side and the lowside. You then switch one side to 12 V to increase and the opposite side to 12 V to decrease pressure -see schematic in reply #20.
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If I was going to do it again, I would just cut the sprayer cable and insert a 12 way deutsch plug between the manual switch box connector and the sprayer, and then create a seperate Section Control box that also used a 12 way deutsch connector. That way you always have the original control box as a backup.
