Water-level controllers are common nowadays. The one described here is built around timer NE555 and inverter buffer CMOS IC CD4049. It uses readily-available, low-cost components, and is easy to build and install on the over-head tank (OHT) to prevent wastage of water.
The circuit works off a 12V battery or 230V AC mains using a 12V adaptor. The three sensors built from non-corrosive metal are fitted to the OHT as shown in Fig. 2 and connected to the circuit (Fig. 1) at appropriate terminals. Power supply terminal Vcc is at the bottom of the tank, sensor terminal L is just above the bottom of the tank and sensor terminal H is at the top of the tank. After you have properly installed the sensors in the OHT and connected the power supply, the circuit is ready to use.