This is the circuit diagram of rain detector alarm. When the sensor is wetted with the water, then the alarm will be activated. This circuit can be used for other purposes.
A 555 astable multivibrator is used here which gives a tone of about 1kHz upon detecting water. The sensor when wetted by water completes the circuit and makes the 555 oscillate at about 1kHz. The sensor is also shown in the circuit diagram.