The project mention here is a versatile moisture sensor alarm circuit. The circuit is quite sensitive and can be used to detect moisture in many things for example in plant soil, wood, irrigation areas, etc. The circuit will produce a beautiful alarm or melody when it will detect moisture content.
Transistor 2N2222 is
working as a switch, when the two probes will detect moisture it will switch on the transistor 2N2222 which will activate the UM66 alarm circuit.