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I would like to trigger a photography/studio strobe flash based on the flash of an iPhone camera's LED flash. I'm picturing a device that I could attach to the back of the iPhone temporarily when I want the system to work. I would like it to respond in less than 500ms.

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  • \$\begingroup\$ And what kind of signal does it take to trigger this strobe? \$\endgroup\$
    – Matt Young
    Sep 5 '13 at 23:33
  • \$\begingroup\$ @MattYoung, good question. I plan on using a trigger cable which I believe simply flashes the strobe on a completed circuit. \$\endgroup\$
    – Kyle Hayes
    Sep 5 '13 at 23:39
  • \$\begingroup\$ @IgnacioVazquez-Abrams That's what he said, but what is the big strobe expecting? Are we talking TTL, or some other voltage levels? A pulse, or just an edge? As it stands, there isn't enough here to even start to suggest a solution. \$\endgroup\$
    – Matt Young
    Sep 5 '13 at 23:44
  • \$\begingroup\$ I would even consider a basic slave AC version: adorama.com/JTS20.html?gclid=CPuTk_G3tbkCFWuCQgodiDYAAw but I think these require a certain range of light to be emitted near by to activate. \$\endgroup\$
    – Kyle Hayes
    Sep 5 '13 at 23:44
  • \$\begingroup\$ Is the IPhone a single flash or does it use pre-flashes to set up exposure? - this determines the type of optical slave you should use. \$\endgroup\$ Sep 6 '13 at 19:49
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Are you sure about 500 ms. That would be a very slow shutter speed. In any case, responding much faster than that is no problem.

Photographic strobes are triggered by a contact closure. Unfortunately the current and open circuit voltage can vary widely, so it is best to use a relay for the general case. You could use a transistor, which would trigger faster, if you know something about the strobe you are trying to trigger.

The question then becomes how to trigger a relay from a light flash. The important characteristic of the trigger flash is that it is a fast pulse event, something that doesn't happen much on otherwise. I'd probably rig up a photodiode into a transipedance amplifier and look for a pulse. This can be done by high-pass filtering and thresholding the amplifier output. When the threshold is detected, turn on the relay for 100 ms or so. That is much longer than a small relay will take to turn on, and much much longer than the resulting strobe flash.

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  • \$\begingroup\$ you would be absolutely right about the shutter speed if this was being used for photography. I won't actually be taking any pictures with this setup. There is an app that uses the LED flash for another purpose that I would like trigger off of. \$\endgroup\$
    – Kyle Hayes
    Sep 5 '13 at 23:46
  • \$\begingroup\$ As I understand it studio strobe flashguns are usually triggered optically not by contact closure. \$\endgroup\$ Sep 6 '13 at 8:50

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