This is a Water Level Indicator with alarm circuit. This circuit is used to indicate the amount of water present in the overhead tank. Not only that, but this circuit also gives an alarm when the tank is full. CD4066 is used to indicate the level of the water through LEDs. Here is the schematic diagram of the  circuit:

Water Level Indicator with alarm circuit schematic
The wires in the tank are open circuited and The 180K resistors pulls the switch low hence opening the switch and LEDs are OFF when the water is empty. When the water is filling up the tank, S1 and the + supply is shorted by water, so the switch S1 and turns the LED1 ON.  As the water continues to fill the tank, the LEDs2 , 3 and 4 light up gradually.

The number of level can be upgraded up to 8, if we use CD4066 ICs. The  base of the transistor BC148 is pulled high by the water and this saturates the transistor, when the water is full. It will turn the buzzer ON. To turn OFF the buzzer, The SPST switch has to be opened. while pumping water, Remember to turn the switch ON, otherwise the buzzer will not sound!