mirith:
mart256:
I would use an Optocoupler. Check the schematic.I wouldn't, since your voltages are going to be sensitive. Reading water like that is a bit odd. You definitely can do it, but the resistances you measure might end up being very different, since the minerals dissolved in the water will affect everything greatly so you could be trying to look for anything between 10s of Megaohms to 100 Ohms. Putting the wires close together (1cm or less maybe?) will help. Make sure to use Tap water to test, since pure water is non-conductive.
When I designed the circuit I was thinking about placing the wires very close together. You are right, if the are far from each other the resistance may be high enough to avoid the optical transistor to be switched.