I built something similar around 1993 out of popular electronics, but is it really that hard to time the exposure manually, especially with digital where misses don't cost you anything?
I shot this using a stock IR remote to trigger the camera as I dropped a quarter in the glass. Overall, I only missed about 20% of the time, and had a nice selection of splashes to choose from.
I found the whole process a lot more fun without the automatic trigger because the setup was much more dynamic.