The people at [Hobby Robotics] decided to build a trigger circuit for lightning photography. There are more complex ways to do this, but they just used a photo transistor and an Arduino. The Arduino watches the photo transistor’s value and compares it to the previously captured one. If the difference is above a certain threshold, it means a rapid change in the amount of light has occurred, which triggers the shutter. An earlier post covered how to directly control the Canon 30d using an Arduino. All of this works because the shutter lag and code execution together are less than lighting’s 100ms duration.