darlington array IC, contains seven darlington transistors and the IC2 is a UM66 CMOS melody IC.

Working of the circuit is simple, the base of all the darlington transistors inside the IC is connected with its pins one to seven that is used as probes, the positive rail is used as the bottom or common probe. When the water level reaches to a probe it will form an electrical connectivity between that probe and the bottom/common probe due to which the associated LED will become activated. When the water reaches to the last or full level probe a melodious alarm will automatically gets activated.
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This water level indicator circuit can sense seven levels of water and shows visual indication by LEDs. The circuit also alarms when the water reaches to the top level. The circuit is using two ICs and few other discrete components.
The IC1 is an ULN2004 that is a seven darlington array IC, contains seven

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Water Level Indicator With Alarm Circuit Diagram