N – Channel MOSFET IRF Z44 N is used as a switch. Buzzer and LED are connected in series with 100 Ohms resistor to the Drain (D) of the MOSFET. Negative of battery is connected to its Source (S). Gate (G) is connected to the Source through 1 M resistor to prevent latching of MOSFET so that buzzer will turns off when the water level drops.
Probe 1 is connected to the Gate of MOSFET and Probe 2 is connected to the out of switch. 9V battery or 12 volt adapter can be used to power the circuit.
Take a glass or plastic jar ( assuming that it is the water tank) . Fix the probes (with 1 cm gap) just below the rim of glass. Connect battery and switch on. Fill glass gradually with water. When the water level touches the probes, buzzer sounds and LED turns on. When the water level drops, probes loose continuity and buzzer stops beeping.