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Unread postAuthor: Insomniac » Wed Jan 28, 2009 6:53 pm

I'll rebuild the circuit when I get two things:

Enough parts to do it.
Figure out how to make the MOC fire the flash... once I do that I'll worry about the rest of the circuit.

Would it be a good idea to get an SCR rated at 400V or so to make a low V and current trigger for the flash? IE have the MOC trigger an SCR to trigger the flash? This article and this instructable do much the same modification in which a neon lamp and an SCR are used to trigger the flash when the neon lamp reaches it's striking voltage.

Would there be a way to wire up a similar SCR so that I would still get two wires coming out of the camera that would fire it when touched, but were more sensitive to the MOC?
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Unread postAuthor: jimmy101 » Fri Jan 30, 2009 12:54 pm

The NE2 trick might work. Most generic NE2 bulbs trigger at about 70V though. You might have to either put a couple bulbs in series or find a neon bulb that triggers at 200 to 300V.

I don't think the SCR will help, it'll probably leak as badly as the triac.

Kind of a long shot but you might just try reversing the polatiry of the MOC's triac leads. The triac should be symmetrical but it almost certainly isn't. You might get lucky and have much less leakage with the triac reversed.

I think the best bet is to either put a mechanical switch between the MOC triac and the trigger cap or reduce the resistance of the 1Meg resistor (R3 in the linked schematics) in the trigger cap's charging circuit.
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Unread postAuthor: Insomniac » Fri Jan 30, 2009 7:41 pm

Hmm. I'll try getting the scr to make the auto-trigger just because it's somthing else I'd like to try... If it works would that mean the SCR isn't leaking too badly?

Oh and the trigger bulb is no problem, I salvaged one from another disposable camera and the electrodes are noticably further apart than on the other little NE2 bulbs I have.


EDIT: What about a small relay? Perhaps that would be fast enough that I could still catch stuff happening. If I'm reading it right that has a response time of 15ms... might just be quick enough, if I shoot at somthing further away.


EDIT: Ok, I did a couple more tests... wired up the moc with a 330ohm resistor and a 9v battery, and checked it was 'on' by attaching a 3v coin cell and a flashing LED... all worked well. Next I tried attaching a joule thief to it, and it wouldn't light. Tried the FLED again and it did, tried the joule thief without the MOC and it lit, then tried it in series with the MOC and it wouldn't light. Odd, I just assumed somthing with the MOC was affecting how the circuit worked and didn't worry about it.


Next I tried charging up the flash, and attaching the trigger contacts to the MOC when the MOC was on and when the flash was fully charged. It didn't fire. This eliminates the problem of leakage as the flash is only connected when the moc is on AND when the flash is charged. When it was attached, I used a bit of metal to short pin 4 to pin 6, and the flash didn't fire (meaning enough current was flowing through the MOC that the trigger cap woudln't charge, but not enough was flowing for the flash to actually fire.)

So then I tried one more test. I disconnected the 9V battery so that the MOC would be in it's 'off' state, and connected the flash to it and charged it. When I shorted pin 4 to pin 6, the flash went off! I take this to mean that the problem is NOT caused by leakage (because the flash could easily keep the trigger cap charged when connected to the MOC), but is caused by the MOC not allowing the cap to discharge fast enough to get the neccisary HV pulse to set off the flash.
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Unread postAuthor: jimmy101 » Mon Feb 02, 2009 9:18 pm

Insomniac wrote:So then I tried one more test. I disconnected the 9V battery so that the MOC would be in it's 'off' state, and connected the flash to it and charged it. When I shorted pin 4 to pin 6, the flash went off! I take this to mean that the problem is NOT caused by leakage (because the flash could easily keep the trigger cap charged when connected to the MOC), but is caused by the MOC not allowing the cap to discharge fast enough to get the neccisary HV pulse to set off the flash.

Interesting. I think you are correct, the triac isn't leaking, it is either retarding current flow too much or it isn't getting triggered by the LED in the MOC.
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Unread postAuthor: Insomniac » Wed Feb 04, 2009 12:26 am

So how would I wire an SCR to be triggered by the triac, and in turn trigger the flash? What specs would the SCR need to cope with the trigger cap?
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Unread postAuthor: starman » Wed Feb 04, 2009 12:41 am

Insomniac wrote:So how would I wire an SCR to be triggered by the triac, and in turn trigger the flash? What specs would the SCR need to cope with the trigger cap?


I'm not completely following what you are doing here, but just to chime in....a triac is really an AC version of an SCR, a DC device. Therefore, triggering an SCR with a triac wouldn't usually be something you would do.
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Unread postAuthor: jimmy101 » Wed Feb 04, 2009 2:32 pm

Take a look at the MOC data sheet. I believe they show a couple examples of the MOC being used to fire an external high power triac. The SCR would be wired in the same way, but since the SCR is polar (unlike the triac) you'll have to keep track of what is the postive and negative leads from the trigger cap.
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Unread postAuthor: ramses » Sat Mar 21, 2009 8:48 pm

Interesting, my disposable camera triggers worked even when shorted through 200ft of AWG 16 extension cord (working on a hybrid), so resistance shouldn't be the issue. You could try putting triacs in parallel, and maybe gating them with a simple transistor.

And BTW, you should consider making the whole setup modular, and getting a Vivitar 283 off eBay. The cut off feature helps greatly with the afterglow and reduces the amount of light. These crops were taken with a setup similar to that of your clothespin (same thing, more duct tape)
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Unread postAuthor: Insomniac » Mon Mar 23, 2009 1:21 am

Hmm... I haven't really bothered with this recently, although I may dig it up again somtime and see if I can make it work.
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