This is one of the simplest water level indicator that can give up to 7 indications – Full, Half and Low water levels in the overhead tank through LED indications. The circuit is simple and uses a single IC and a few components.
The main part of the water level circuit is the 7 channel Darlington array IC ULN2004. Its seven inputs are connected to seven probes to sense the water level and the corresponding outputs are connected to three LEDs through current limiting resistors. A common probe (common) is connected to the positive rail. When this probe gets electrical continuity through water with the other probes the corresponding LEDs light.
Here is an idea how to connect the 7 segment display.
Assemble the circuit on a common PCB and enclose in a small box. This unit can be kept in a place for easy monitoring. Use brass or chromium plated pins as probes. Connect the probes with the unit using three core shield wire. Probes should be plugged into the tank through the side. The low probes in the lower position, half probes in the middle positions and high ones in two upper position. If the tank is empty, no LEDs will be on. When the water level increases the green LEDs (marked with G) will start to light up according to the H2O level. When the tank becomes full the red LEDs (marked R) will be on. In short, if Green, Yellow and Red LEDs light, tank is full.