Arduino DSLR flash trigger. Laser shines on a light sensor. And interruption of the beam is detected by a decrease in intensity, which triggers a flash after a specified delay. schematic: http://www.flickr.com/photos/cbonsig/5478376580/ photos: http://www.flickr.com/photos/cbonsig/tags/trigger/
Based on code from: Maurice Ribble http://www.glacialwanderer.com/hobbyrobotics