Needless to say, there’s a world of difference between the expensive DSLR cameras professional photographers use and the point-and-shoot models carried by commoners. One such difference is the ability to use slave flashes – a second flash set off to the side of the subject for better illumination. Most of these slave flash units are triggered when they see a bright light, or when the on-camera flash goes off. Point and shoot models usually have a ‘pre-flash’ that cause a slave flash to trigger prematurely. [Kerry] built a really neat slave flash that is able to work with these point-and-shoot cameras, and is pretty easy to build as well.

There are two options when it comes to building a flash that can work with a point-and-shoot: First, measure the time between the pre-flash and real flash, and then simply delay the slave flash. This option has a few problems. Even when [Kerry] tested this technique on the same camera, the delay between the flashes were never consistant.

The second option is to simply ignore the pre flash and synchronize with the main flash. This is a little harder, but if done right this technique is nearly foolproof.

[Kerry] ended up building a small circuit out of a 556 timer chip and an LM339n comparator that turns itself ‘on’ just a little bit after it sees the pre flash. From there, the device looks for the main flash and triggers itself whenever it sees another bright source of light.

The finished product works beautifully, and is simple enough for just about anyone to build on a piece of perf board.