This is one of the simplest water level indicator that can give three indications – Full, Half and Low water levels in the overhead tank through LED indications. The circuit is too 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 ULN 2004. Its three inputs are connected to three probes to sense the water level and the corresponding outputs are connected to three LEDs through current limiting resistors R1 through R3. A common probe (probe A) is connected to the positive rail. When this probe gets electrical continuity through water with the probes B, C and D corresponding LEDs light.
Position of probe
Probe A Bottom of tank
Probe B Upper position of tank
Probe C Middle position of tank
Probe D Lower position of tank
Simple Water level Indicator Circuit diagram
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- Probe A and D in the lower position, Probe B in the upper position and Probe C in the middle position. If the tank is empty, only the probes A and D get current through water so that Red LED only lights. When the water level increases to half of the tank, Probe C gets continuity and Yellow LED also lights. When the tank becomes full, Probe B gets continuity and Green LED lights. In short, if Green, Yellow and Red LEDs light, tank is full. If Yellow and Red LEDs light, tank is half and if Red LED only lights, tank is empty.