Here is a working design for a water level indicator and controller using the concept of the common emmiter configuration.
The circuit utilizes BC148 transistor, having LED indicators for different levels of water in the tank. A relay is connected to the top most level indicator. The water motor is powered via the Normally connected and common terminal of the relay.
As soon as water reaches the top level base biasing is available and the relay is energised, the common terminal of relay gets connected to normally open and at the same time disconnecting the motor, avoiding overflow of water from overhead tank.
Though simply using the single transistor can get the work done, but multiple CE configurations are used to display different levels of water.
For sensors you can simply use single strand copper wires used in house wiring.
I have tested the circuit and it works fine with wires upto 10 meters long.
Circuit diagram created using http://www.gpleda.org/
Modification for relay latching.
The previous circuit has a little problem i.e. if some one does not switch off the device completely, the motor will keep of switching on as soon as there is a little drop in the water level.
To get rid of this problem we need to latch the relay, i.e the relay coil should keep getting power even after the water level decreases from the top level and top level transistor responsible for triggring the relay is disabled.
To do this we need a DPDT relay, here one set is as ususal going to be used to disconnect the motor.
The relay coil's one point is as usual connected to positive supply. The second is connected to common which goes through the Normally Connected to the Top level transistor.
The normally open is connected to ground.
As soon as water level reaches the top the relay will be triggered, immideately the second terminal of relay connected to common will now get connected to normally open. This will make sure that the supply now is be given to the relay directly from positive and negative directly and not via the sensing transistor.
So now even if the the level goes down the relay will be energised, untill someone switches off power supply to the circuit.
If required a simple push to off switch can be connected between the relay terminal connected to positive. A momentry press of this switch will reset the relay.
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