In an early version of the camControl device (before the dropController) I used an interrupter/optoisolator to detect the water drops. The plan was to detect the water drop, wait a little bit and then activate the shutter.
There are various different kinds of photo interrupter, different shapes and different sizes but all do the same job.
A photo interrupter has a LED at one side (normally IR) and a photo transistor at the other. When the LED in emitting light the photo transistor allows a current to flow. Remove the light and the current stops.