I thought it would be interesting to add a rain sensor to my tipping bucket rain gauge. Because it take time for the bucket to fill and does not always tip right when it stops raining a sensor that detects rain would help tell more precisely when the rain starts and stops.
The Hydreon RG-11 rain sensor looked like it would work well for this application. This sensor is rugged and not too expensive. It works on the same principle as automatic windshield wipers. The rain drops on the clear plastic reduce the internal reflection of light. It can be set to just detect rain or measure rain fall. Although it is not terribly accurate at measuring the amount or rain fall. I set it to just detect rain. The sensor runs on 12 volts and outputs a contact closure when it detects rain.
The RG-11 sensor is the dome to the right in the picture.
I already have a tipping bucket rain gauge connected to the Internet so it was fairly simple to add the RG-11 to the setup. The sensor was connected to an input on the SparkFun ESP8266 DEV and the software modified to read the input and report it to my web server. I also had to modify the web software to log the readings to the SQL server and add the information to the plot on the web page. See my IoT rain gauge article for more information.
Just the day after I installed the rain sensor it rained on the last day of December. The tipping bucket data is the blue line on the plot and the green areas are when the sensor is indicating rain. In the plot above the green line around 8:00 is when I manually activated the sensor to test it. Around 16:00 the precipitation gradually changed to snow. The sensor does not detect snow.
http://rainsensors.com/ Hydreon RG-11 rain sensor
https://jimhannon.wordpress.com/2018/02/02/iot-rain-gauge/ My tipping bucket rain gauge installation.
http://www.ocrslc.net/sensors/rain.php The sensor online.