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A 'Smart' Slave Flash Trigger

Does your camera's flash operate in red-eye reduction (multiple flash) mode only? This clever unit counts the number of "pre-flashes" before triggering a slave flash unit.

By Jim Rowe

MOST OF THE LATEST digital still and film cameras have a built-in electronic flash, which at first glance seems great. The trouble is that it's almost impossible to take a good professional photo with only a single flash. They're OK for "happy snaps" but that fixed flash, right next to the lens and pointing in the same direction is a big problem. It gives very "flat" lighting and very dark shadows.

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Fig.1: the camera flash is picked up by photodiode PD1 and this drives transistor Q1 which in turn clocks IC1. IC1 is wired as a programmable counter and the output of gate IC2c (pin 10) will go low only when the right number of pulses have been counted. IC2c then triggers SCR1 (via IC2b & Q2) to trigger the slave flash unit.

For much better modelling and control of shadows, you really need at least one additional source of light and/or a system of light diffusion. But neither of these options is easy with most digital cameras, not only because of their fixed forward-facing internal flash but because they generally don't have a "hot shoe" or conventional flash contact socket to trigger an external flash.

So the only way to trigger a second flash with these cameras is to use a slave flash trigger unit. This has an optical sensor which detects when the camera's own flash operates, to trigger an external "slave" flash.

But there is a further complication with many new digital cameras. Their internal flash often operates only in "red-eye reduction" mode, where the flash gives not just one single pulse of light but multiple flashes. There may be one, two or even a bunch of short pre-flashes shortly before the main flash.

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