[Greg Lipscomb] was working on this disposable camera based slave flash when he stumbled into his macro photography project. Slave flashes are used as fill lighting and can be triggered by several different methods. Greg’s project uses a photocell and a microcontroller for trigger and timing. It also makes sure the flash stays charged. He concedes that this design is a bit complicated, but he went with it because he didn’t have any silicon controlled rectifiers available. The microcontroller would be too slow, but his Canon 10D uses a pre-flash before the actual photo, so the slave has a built in delay from that first flash.