Many older flashes lack for a jack to input a sync cable and rely exclusively on a simple slave mode triggered by the primary flash. This hack uses a piece of scrap fiber optic cable to trigger the flash in bright conditions.
Using a flash as an optical slave indoors isn’t much of a problem, but if you introduce bright light (such as outdoor lighting conditions), the ambient light can overpower the small on-camera flash and render the optical slave function useless. To overcome this, Marcell over at Fiber Strobe (a blog dedicated to cataloging experiments in incorporating fiber optics into photography) came up with a simple work around. By using some foam crafting materials and tape, he whipped up a simple mount for a strand of scrap fiber optic cable to connect between the on-camera flash and the sensor on the slave flash. Once attached it works exactly like as sync cable would, except it’s transmitting a pulse of light instead of a pulse of electricity.
Hit up the link below for more pictures and a build guide.