Rain Drop Sensor Module for Arduino Raspberry PI is a Highly rated product. Make your own automated cloth hanger with rain alarming. 

1 This sensor adopt high quality of RF-04 double sides material.Area of 5.0*4.0CM,plate nickle on side,Anti-oxidation,anti-conductivity,with long use time;
2 the comparator output signal clean waveform is good, driving ability, over 15mA;
3 Potentiometer adjust the sensitivity;
4 Working voltage 3.3V-5V;
5 Output format: Digital switching output (0 and 1) and analog voltage output AO;
6 With bolt holes for easy installation;
7 Small board PCB size: 3.2cm x 1.4cm;
8 using a wide voltage LM393 comparator;
 
Instructions:
 1 Connect 5V power supply,Then power indicator light.No water on sensor board,DO output shows "high level",the switch indicator light off..Drop a water,DO output shows "Low level",the switch indicator lights.
2 Clean the water,it returned high level status;
3 AO analog output,could connect MCU 's AD port to test the size of rainfall.
4 DO TTL digital output also could coonnect MCU to test if there has raindrop.
 
Mode of Connection:
 1 VCC: positive power supply (3-5V);
2 GND: tied to power supply;
3 DO: TTL switching signal output;
4 AO: analog output.