Fritzing

The circuit is designed to indicate three levels of water stored in the tank: low but not empty, half and full but not overflowing.  

• When there is no water in the tank, all the LEDs are off as an indication that the tank is completely empty.
• When water level increases and reaches M2, the contacts M1 and M2 get shorted as water acts as a conducting medium between M1 and M2.
• This will turn on the transistor Q1 and the Green LED starts to glow. As the water level continues to rise and reaches half the tank, M3 will come into contact with water and receives a small voltage from M1.
• As a result, Q2 is turned on and Yellow LED will glow. When the water in the tank rises to full tank, M4 is also shorted with M1 and both Q3 and Q4 will turn on.
• The Red LED glows as an indication that the tank is full.