This water-level indicator-cum-alarm circuit is configured around the well-known CMOS input-compatible, 7-channel IC ULN2004 Darlington array. As the water level rises in the tank, it comes in contact with probes P1 through P7 and thereby makes pins 7 through 1 high, sequentially.
As a result, the corresponding output pins 10 through 16 go low one after the other, and LED1 through LED7 light up in that order. When water comes in contact with the final probe P7, it results in sounding of the piezo-buzzer connected to output pin 16 along with LED7.
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