S1,S2,S3 and S4 are the four probes for different positions of water tank. When any of the probes touches the water circuit will be closed and voltage coming to the base of the transistor(the base current Ib get increased by current passing through the water), LED goes ON. So easily we can understand water in the tank is in corresponding water position. 100k Resistor is used to pull-down the base of the transistor(Not to produce false result). If direct Vcc voltage coming into the base of the transistor,It may affect the transistor(So 1k is provided series to the base to limit the base current to micro amperes ).

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