When water in the tank touches the probe A and B both, a small current flows from A to B through water and to the base of transistor T1 via a 220KΩ resistor. As a result the transistor conducts causing the LED1 to glow
. Similarly, when water touches sensor C, LED2 glow
and indicates that WATER is at minimum+ and when water touches sensor D , LED 3 glow
and indicates that WATER is at. MIDDLE . Finally, when the water touches sensor E , BUZZER
turns on and indicates the tank is completely filled.