The first thing is to make a rough circuit for triggering the camera shutter and the flash, as I said earlier I had previously done this with a transistor and a relay but after reading a few posts about water drop photography it became very clear that a relay just isn’t fast enough due to its mechanical nature.
Transistors on the other hand are very fast switching and so they are perfect for the job. I wanted to keep the camera and flash trigger separate from the Arduino as the high voltages that the flash produces could damage the Arduino, and I plan to do this with an ‘opto-isolator’. An opto-isolator is basicly a LED and a Photo Transistor in a DIL package, turning the LED on will cause the transistor conduct but there isn’t any electrical connection between the 2 devices which means that the transistor is isolated from the rest of the circuit. Here is a very simple diagram that should get you started, you’ll need 2 of these, one for firing the shutter and one for triggering the flash guns; obviously the switch is just there to simulate an Arduino output pin.
The next circuit to work on was the sensor which would detect when something has passed through it; again this is a simple diagram. I have shown an opto-coupler in the circuit below, the advantage of an opto-coupler is that it is already assembled in a package is there is no need to align the LED and Photo Transistor; however the disadvantage of an opto-coupler is the very small working distance. Another method for a sensor could be to use a laser point and an LDR (light dependant resistor), this will allow a much greater working distance; but aligning the 2 devices can be tricky and the LDR will be effected by ambient light!
For consitant water drops the best thing to use is a solenoid valve, however these require a minimum of 12v and also draw more current that an Arduino pin can supply. We can still use the Arduino for controlling the solenoid, and this can be done by using an output pin to drive an opto isolator (as in the circuit above) which will turn on a FET and activate the solenoid, the idea of using an opto isolator to drive the FET is to protect the Arduino board from any harmful back EMF from the solenoid. Below ciruit that I got from the tutorial section on the Arduino website, I have made 1 or 2 modifications as the one on thier website didn’t work in the way that I wanted it to!