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 | |
 |
$8.77 End Date: Sunday Sep-8-2019 17:51:46 PDT Buy It Now for only: $8.77 |
| Rain Water Level Sensor Depth Detection Module For Arduino Liquid Detector Drop | |
 |
$1.31 End Date: Monday Sep-23-2019 7:15:39 PDT Buy It Now for only: $1.31 |
| Rain Water Level Sensor Module Detection Raindrop Humidity Arduino Depth Board | |
 |
$6.74 End Date: Saturday Sep-14-2019 22:34:36 PDT Buy It Now for only: $6.74 |
| CB7 Snow Raindrop Sensor Detector Module Rain Humidity Module for Arduino New | |
 |
$2.45 End Date: Saturday Sep-21-2019 20:02:09 PDT Buy It Now for only: $2.45 |
| Sensor Rain Sensor Arduino Detector Module HD Rd Snow Leak Water Weather | |
 |
$5.69 End Date: Monday Sep-23-2019 8:21:17 PDT Buy It Now for only: $5.69 |
| Rain Water Level Sensor Module Detection DC 3V 5V Liquid Arduino Depth Board | |
 |
$5.39 End Date: Saturday Sep-14-2019 20:13:45 PDT Buy It Now for only: $5.39 |
| Arduino Rain Sensor Module Rain Drop Detector Relay Control Module 12V DIY | |
 |
$3.25 End Date: Monday Sep-23-2019 4:30:36 PDT Buy It Now for only: $3.25 |
| Arduino Rain Sensor Module Rain Drop Detector Relay Control Module 12V | |
 |
$4.74 End Date: Wednesday Sep-18-2019 8:21:07 PDT Buy It Now for only: $4.74 |
| Rain Raindrops Snow Sensor Module Detector Pour Suspension Water Wetness Arduino | |
 |
$5.45 End Date: Wednesday Sep-4-2019 4:40:00 PDT Buy It Now for only: $5.45 |
| CB7 Snow Raindrop Sensor Detector Module Rain Humidity Module for Arduino New | |
 |
$2.63 End Date: Saturday Sep-21-2019 19:57:48 PDT Buy It Now for only: $2.63 |