The floating bulb on a stick idea sounds good. I suppose one could get a sensor that they have in automotive tanks that causes a light to go on when gasoline gets below a certain point. They work by a thermocoil. Normaly the liquid cools the coil, but when the liquid gets below the coil, the coil heats up and closes the circuit. Instead of connecting it to a light, you could connect it to a buzzer. I've bought buzzers that run on 12 volts at Radio Shack. The thing could be powered by a 12 volt transformer using household current.
[Non-text portions of this message have been removed]
