Some models come with a basic analogue to digital converter that's adjustable. It requires a 3 volt and ground connection, and has a signal line which triggers when wet.
I wired the signal connector directly to a GPIO. It may also run at 5volts but didn't want to risk the pi.
| 2pcs Rain Sensor Detector Module Raindrop Detection Shield Arduino Compatible US | |
 |
$8.49 End Date: Monday Nov-6-2017 16:51:46 PST Buy It Now for only: $8.49 |
| 1 Rain detector module and sensor board kit for easy arduino interface DIY DG | |
 |
$0.99 End Date: Friday Nov-10-2017 0:39:52 PST Buy It Now for only: $0.99 |
| 1 Rain detector module and sensor board kit for easy arduino interface DIY 2O | |
 |
$0.99 End Date: Thursday Oct-26-2017 20:20:08 PDT Buy It Now for only: $0.99 |
| 1Pc Hot Rain Detector Module Sensor Board Kit for Easy Arduino Interface DIY | |
 |
$0.99 End Date: Friday Nov-10-2017 3:01:58 PST Buy It Now for only: $0.99 |
| 1 Rain detector module and sensor board kit for easy arduino interface DIY HW | |
 |
$0.99 End Date: Friday Nov-3-2017 17:54:51 PDT Buy It Now for only: $0.99 |
| RAIN DETECTOR BOARD FOR ARDUINO AND COMPATABLES SHIPS FROM US | |
 |
$0.99 End Date: Thursday Nov-9-2017 7:28:56 PST Buy It Now for only: $0.99 |
| CB7 Snow Raindrop Sensor Detector Module Rain Humidity Module for Arduino New | |
 |
$2.58 End Date: Monday Nov-13-2017 17:07:13 PST Buy It Now for only: $2.58 |
| New Raindrop Detection Sensor Snow Rain Detector Weather Module Humidity Arduino | |
 |
$0.99 End Date: Saturday Oct-21-2017 1:21:20 PDT Buy It Now for only: $0.99 |
| Raindrop Detection Sensor Snow Rain Detector Weather Module Humidity For Arduino | |
 |
$0.99 End Date: Friday Oct-20-2017 8:43:15 PDT Buy It Now for only: $0.99 |
| Rain Drip Detector Module Shield Sensor Cable For AVR ARM Arduino Compatible | |
 |
$1.99 End Date: Friday Nov-17-2017 19:26:55 PST Buy It Now for only: $1.99 |