The adjustment is made to limit the low pH when the solenoid is on. The range in pH is determined by the resistance ratios of the schmidt trigger, which are the 1M, 220 and 619K resistors attached to the non-inverting (+) input of the IC labelled A3. The 220 ohm is the key. The lower the ohms, the tighter the control. The problem is that you get to a limit which defeats the purpose of the circuit. The circuit adds or subtracts a little bit of voltage so that when the relay turns the soleniod on or off, it's outside the noise level and it stays on or off - no chatter. The values I have selected add about 3 mV to the set point, where 1 mv is 0.01 pH units. When it turns off, e.g., at 6.85 and the instrument stabalizes in a minute, the pH reads 6.82. It turns on again at or about 6.93, sometime higher.
When I get some time, I've got to update the schematic and add some user notes. I'll try to assemble some sort of price list to buy the components. I'll not bother to offer any great detail about thing you can buy at Radio Shack but I will include general prices. You can get the panel meter from Circuit Specialist. The best and only place a hobbyist can buy all of the cute little IC that don't take kindly to shorts circuits, reversed polarity and other stupid assembly methods (yes, that's experience talking) is DigiKey. Although they do not have the lowest prices on basics compents like resistors, they are inexpensive and the IC prices are reasonable (about $3 for the fancy chips). You will appreciate their customer service: prompt delivery and an inventory so big it's hard to find the part you want. Plus, there is no minimum order. They have a small service charge for order below $25 and you only pay exact USPS express mail changes, which cost you only several dollars. It's one stop shopping for everything but the panel meter.
After all of the screw-up along the way, I don't want to build anymore proto-types, at least for a while. I just want to watch and listen to it work.
Andrew, MASI Treasurer
This message is always under construction: 75-gallon tank; 2, Eheim 2026 filters - one twice broken; Tek Light with 4, 54W T5s (6000K) ; Sand on top of 4:1 sand:clay mixture; Milwaukee CO2 controller; PlantGuild vortex CO2 reactor; pH = 6.6, kH=70mg/l, GH=120mg/l; EI; Flourish excel on 50% weekly water change: AGA Member.