Here is a very interesting project of a 555 rain detector / sensor alarm circuit. The circuit will produce a melody when the sensor will detect any water drops on its surface. The heart of the circuit of this rain sensor alarm is a famous 555 timer IC. The melody circuit is built with the UM66 melody IC.
A single transistor amplifier is used at the output of the UM66 IC for amplifying the audio output to drive an 8 ohms speaker. An LED is also used to show the visual indication of raining.

The circuit is using a hand built sensor, it can be made by many techniques for example you can make it simply by attaching thin wires on a 3 to 6 inches square plastic piece, make sure to give 1 to 3 millimeter gap between all wires. The max operating voltage of the circuit is 4.5 volt DC. For operating with higher voltage like 6, 9 or 12 volt a zener diode of 3.3V with a resistor will be required before the UM66 IC. 

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555 Rain Detector / Sensor Alarm Circuit Diagram