Connect the metal pins on the water tank as one at the bottom, one at the minimum level , one at maximum level and last one at the point at which the water will overflow.
Pin A is connected to +ve of battery.3 LED are showing the amount of water in the tank
The red LED(low) will be always on.
The fact is that the water is a conductor. If the level of water is between C and B, +ve charge from A will pass through B. In this way the base of t2 gets +ve charge and t2 turn on.this will turn on green led.
When the water reaches B t2 turns on and Orange LED glows.
If Water level is lower than B the t3 turn on and the siren for low water level makes sound.
If water reaches D then the t4 switches the siren for high water level.
This will be very useful to every houses.