Thanks. Like I said, shunting that potentiometer is not of any practical use. It all started as an idea when I was playing around on my breadboard trying to build a liquid detector. I'm probably doing it all wrong OR over-engineering it and although I HAVE consulted some other design ideas beforehand, I thought "hmmm.....I can probably make it better"
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You'll have to excuse any errors that follow, since this is from memory right now:
Fig.1 in this attachment: I started by using just a comparator IC alone.
I set a reference voltage on its N/I pin using a trimmer R3.
I then used a resistor divider (R1,R2) on its inverting input and adjusted the trimmer R3 so the voltage there is just a hair below the inverting input (can't remember exact values here, so please ignore that for now).
I connected my detector pad/cell between the inverting input and GND, in parallel with the divider I just created.
The idea is that when dry, the cell is open circuit and doesn't interfere at all, since all this is is just two traces zig-zagging close together, but not actually touching. When a liquid falls on the traces, it unavoidably bridges the tiny gaps between the traces, acting as a new resistor to GND, parallel to R2.
Since that input is already sitting just a smidgen below the N/I input, the drop in resistance to GND is just enough to upset the voltage set by the divider, which triggers the comparator.
I adjusted the trimmer R3 so that the cell doesn't cause a reaction when I touch it with my fingers, but reacts when splashed with water for instance.
I used a NPN on the output of the op-amp to switch a relay (not shown).
I then took it one step further and added a resistor from the output back to the inverting input for some hysteresis, as thought by.....you guessed it: Dave himself
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This seemed to work well enough for my purposes, even in this half-arsed manner, but was sub-optimal because using a trimmer to set a reference is anything but stable. Sure enough, if I played around with the input voltage I was feeding this thing, the relay came on when my finger was touching the cell, even though it wasn't doing that before, which makes perfect sense, since I "tuned" my trimmer with a Vcc of 12v, so decreasing Vcc also threw off my reference. The power supply for this project CAN sag considerably, since it can run on battery if mains fails (alarm system), so a voltage as low as 10.5v might be expected, even if pretty unlikely.
I was going to use a zener to set the reference voltage (not sure what voltage would be appropriate), but I learned a 431 might be even more stable, plus it's adjustable and I'm not limited by physical space or part count, so I went with that. I set it up on the breadboard, still using a trimmer to mess with the REF pin, as shown in the first post, Fig.1. It seemed to be very stable, even with my humble DMM. I was going to feed the reference voltage into the comparator and still use the resistor divider idea on the opposite input. I then began thinking how I'd go about attaching my moisture cell and NOW you finally understand how this topic came to be: I thought "what if I connect this here, just across the pot like that ?" and then began speculating what would happen if I just short across my cell entirely.......for some reason - say there's a mercury leak
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- so there you have it...Load of baloney, I know....
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I'll keep experimenting with this, see where it gets me...sky's the limit, ain't it ?
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