Very simple, but very effective design using only a few components. Based on an ETI design. This kit activates a second flash unit when the master, or camera mounted, flash unit is activated. This is useful to fill in shadows and improve the evenness of the lighting. It works by picking up the bright flash with a phototransistor and triggering an SCR. The SCR is used as a switch across the flash contacts. This circuit does not false trigger even in strongly lit rooms, but is sensitive enough to operate almost anywhere within even a quite large room. Of course, by making more of these and fitting them to more slave flash units even better lighting and more shadow reduction is obtained. PCB: 21 x 21mm.