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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 20 May 2006 (Saturday) 14:29
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optical trigger

102 posts
Joined Jun 2003
May 20, 2006 14:29 |  #1

I just bought an optical trigger to use with a 420EZ flash. It will fire once properly if I fire a flash with my 550EX. After that, though,it does not fire again unless I turn the flash off or take the trigger off and put it back on. Am I missing something? I have tried all the setting I know (full manual) etc. Is this how they are supposed to function?

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Cream of the Crop
26,853 posts
Likes: 20
Joined Feb 2003
Location: ABE, PA USA
May 20, 2006 17:00 |  #2

Nope, you're not missing anything. Evidently, the Canon Speedlite flashes have some current flowing through the hotshoe after it's been fired. Other flashes that work fine with optical triggering units do, too, but the current flowing through those is not as high as that of the Speedlites. The characteristic of an SCR (Silicon Controlled Rectifier, which is the component in the trigger unit that basically acts as an electronic switch turned on optically) is that once it is triggered, it will stay turned on for as long as the minimum holding current required to keep the SCR turned on is flowing through it. So, the net effect of the higher current flowing through the Speedlite hotshoe is that the SCR does not turn off. If it doesn't turn off, then you can't trigger it again to fire the flash until you disconnect and then reconnect the flash or turn off the flash and then turn it back on again.

I just spent an hour and half this afternoon with my Speedlite connected to my home made optical trigger with a digital multimeter connected across the sync wire and ground (to mearure voltage) and an analog VOM connected in series with the sync wire (to measure the current through the sync wire). That's how I came up with the above observation.


102 posts
Joined Jun 2003
May 20, 2006 17:06 as a reply to  @ PacAce's post |  #3

wow, thanks. I guess all optical triggers won't work then. I was going to try one of the more costly ones (i have a$14 ebay one). Which is a shame as I think that was the only peice of the puzzle keeping me from going pro :)

Curtis ­ N
Master Flasher
19,129 posts
Likes: 4
Joined Apr 2005
Location: Northern Illinois, US
May 20, 2006 22:19 |  #4

See if you can pickup an old flash unit at a camera shop, pawn shop or wherever. Better if you can get one with variable manual settings. They usually go pretty cheap and most will work fine with a slave trigger.

"If you're not having fun, your pictures will reflect that." - Joe McNally
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116 posts
Joined Oct 2005
Location: Santa Clarita, CA
May 21, 2006 03:32 |  #5

I had the exact same problem with my 420 and optical triggers, fortunately I have several old flashes that will work just fine with them. Still, I wish the 420 would work, but at least thanks to PacAce's description above, now we know why they won't work more than once. That was the part that always confused me, why they would fire one time correctly, but never again unless you turned it off and then back on.

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Harcourt Fenton Mudd--- I, Mudd

Cream of the Crop
6,900 posts
Gallery: 3 photos
Likes: 44
Joined Jun 2005
Location: Munich, Germany
May 22, 2006 07:05 |  #6

Metz has an optical trigger that works.
I tried it out at my local camera store a few weeks ago and it worked perfectly. Other tests with Hama triggers didn't work.

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optical trigger
FORUMS General Gear Talk Flash and Studio Lighting 
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