Just a couple of transistors are enough for implementing a simple water level alarm circuit and used for getting a warning signal when the water level inside a tank nears the overflowing level. The two transistors are configured as a high gain, high sensitive switch, which also is capable of generating a tone when the shown terminals get bridged through the terminals coming in contact with the water inside the tank. The water offers just about the right resistance value across the specified points of the circuit for initiating high pitched tone or the desired warning alarm.
Transistor BC547, 2N2907
Capacitor 0.02microfarad to 0.06microfrad
9volt battery + battery cap