You will need to make some modifications in order to transform this power on delay circuit into a auto power off circuit: invert the inputs of the op-amp. The formula is the same. You can use LM339 or LM393. It depends on what you want.
Kchan, what do you want to accomplish? Do you want the pump to turn off when the water reaches a certain level? What sensor(s) do you have? Cumesoftware's suggestion will work for a time-based turn-off, if that such what you are seeking.
@cume sorry for the poor information i give, i already have an auto on/off timer based switch but it doesnt function well as sometimes the water is weak and the tank wont be full in a time period i set.
@thing yes thats what i wanted, but i dont have any sensors and etc yet. i also know only basic in electronics so i hope if u guys have a schematic, the components wouldnt be so hard to find coz i dont know how to get replacements parts
These will work for fish tanks, cisterns, et al. You might want to double-check temperature tolerances if your tank is a hot-water heater. The same company has float switches for as little as seven dollars.
Aquahub has many competitors; a little shopping around should net you just what you want. Try the search engine term "float switch."
Normally float switches are used for such applications. They are more reliable and durable, and yet simpler than electronic sensors. It is pretty common to find one in water tanks and reservoirs. It is better than a timer, since it relies on water level. I didn't know what you were looking before, so, sorry for my other suggestion.