This circuit will emit an intermittent beep or will flash a LED when the water contained into a recipient has reached the desired level. If a deeper sensing level is needed, the clips can be extended by means of two pieces of stiff wire.
http://www.redcircuits.com/Page103.htm
By means of a Relay, employed to drive a water pump, this circuit provides automatic level control of a water reservoir or well.
http://www.redcircuits.com/Page129.htm
A simple device to indicate various levels of hot water in a tank.
http://www.zen22142.zen.co.uk/Circuits/Misc/hwtank.htm
This circuit not only indicates the amount of water present in the overhead tank but also gives an alarm when the tank is full.
http://www.electronic-circuits-diagrams.com/alarmsimages/alarmsckt6.shtml
can be used to also measure almost any liquid.
http://www.imagineeringezine.com/PDF-FILES/capgage.pdf
uses an ac-sensing signal to eliminate electrolytic corrosion on the probe.
http://www.uoguelph.ca/~antoon/circ/sensor3.htm
uses bead thermistors sticked to to the tank as sensors
http://www.mitedu.freeserve.co.uk/Circuits/Misc/hwtank.htm
capacitance type
http://www.discovercircuits.com/PDF-FILES/capgage.pdf
I n most houses, water is first stored in an underground tank (UGT) and from there it is pumped up to the overhead tank (OHT) located on the roof. People generally switch on the pump when their taps go dry and switch off the pump when the overhead tank starts overflowing. This results in the unnecessary wastage and sometimes non-availability of water in the case of emergency.
http://www.electronicsforu.com/electronicsforu/Lab/freecircuitslist.asp?id=247
Here is a simple, versatile cir- cuit which indicates the level of water in a tank. This circuit produces alarm when water level is below the lowest level L1 and also when water just touches the highest level L12.
http://www.electronicsforu.com/electronicsforu/Lab/freecircuitslist.asp?id=48