There is now an updated and more detailed post. See Controlling a Solenoid Valve from an Arduino. Updated.
Using the Arduino to control the solenoid valve is simply a case of setting a pin high for the appropriate amount of time. There is, however, a caveat, the solenoid works at a different voltage to the Arduino and you cannot directly connect the two. In this case a TIP120 transistor is used as a bridge.
The TIP120 allows a small dc voltage (from the Arduino) to switch a larger dc voltage (12V to the solenoid). It can be thought of as a switch, applying a current to B allows current to flow between C to E.
Solenoid valves are used to control the flow of liquids and gasses and they are an ideal way to create water drops. To get started I purchased a small range of different solenoids with the intention of testing them out and seeing which was the best.
There are several ways you can connect an Arduino to a DSLR so that you can use the Arduino to activate the shutter. I chose to use an optocoupler, sometimes called an optoisolator.
There are many types of optocoupler and you chose one based on the requirements of your circuit. My circuit is a 5V Arduino and a Canon 40D which has about 3.2V on the shutter release. Due to the relatively low voltages there are many suitable optocouplers to pick from. I already had a Fairchild 4N26 so this is the one I used.
Optocouplers are digital switches. They work by using an LED emitter paired with a photo detector transistor. This means they can be used to allow one circuit to switch a separate circuit without having any electrical contact between the two. Basically, if you put a current through pins 1 and 2 and light the LED the photo detector transistor detects the light from the LED and allows a current to flow through pins 5 and 4. No current on pins 1 and 2 means current does not pass through pins 5 and 4.
For quite a while I was looking at water drop photos and thinking about trying to do them myself. I finally gave it a go. My first try was simply making drops and trying to capture the splash. Everything was manual; the water drop, the shot, and it was very hit and miss. Mostly miss…
I did get some shots I liked but these are nothing compared to the shots from other people.
When researching online it quickly became apparent I wasn’t going to get better shots with the setup I had; which was a plastic bag with a small hole and hitting the shutter release like a maniac.