My main concern is the current required to close the relay.
Perhaps a 3rd party flash for the DSLR will show the trigger current or possibly measure it as its likely much lower.
Second is the duration of the sync on the camera, will it be closed long enough to trip the relay?

I am really getting confused. Look at this thread here:

https://photo.stackexchange.com/questions/93615/what-is-the-pc-port-amperage-trigger-limit-for-a-canon-dlsr

It really contradicts what I have been told elsewhere. Here is part of what was said:

"I can't say with certainty what Canon used in the 5D Mark III, but according to a discussion thread on Electronics point, the XT uses a 2SK2715 MOSFET for the flash trigger, and it's probably a good bet that they use the same part on all their cameras.

The 2SK2715 is made by ROHM Semiconductor, and according to the spec sheet, it is rated at 500V at up to 2A. Canon downrated their maximum spec to 250V, either as a hedge against slightly substandard parts or to allow them to source other equivalent parts that may have looser tolerances.

Either way, a part designed to handle up to 1 kilowatt triggering power laughs at your 0.3395 Watt triggering current requirements, and any even-semi-plausible replacement MOSFET that handles such high voltages will have similar specs. You should be fine."

My experience has been in repair, not design. I have identified failures that were a direct result of design flaw and what was happening so that a solution could be found.
Electronic components work best/last longest when their normal load is less than 1/2 of the rated capacity. I would not rely on the spec sheet of a component for the max load one could put on a circuit.
One would need to see the rest of the circuit and its configuration to know what the limits are.
Modern electronic flash still use 350 V to 425 V to the flash tube and in the event of a catastrophic failure a high voltage and or current surge can be felt at the trigger contacts.
I'm a Nikon/Large format person and Nikon recommends using their brand only.
The reason a manufacturer recommends their brand accessories is their designs will not overload their cameras if a catastrophic failure occurs.

It will be at least 10 to 15 days before I'm free enough to do any flash testing.

Hi All,
Not sure if anyone will read this old post. I have now spent many months trying unsuccessfully to solve this problem. I have studied electricity. I have probed individuals on every electrical support group I can find. I have even hired an actual electrical engineer. All have failed to produce a workable solution. The person I hired was not local, and I believe that may have contributed to the inability to resolve the issues. So, I have given up on this effort for now. Sorry if this post is not appropriate.

I would like to hire another electrical engineer but only locally this time. I live in the Kansas City Metro area. I think it is best to be someone that I can physically see and that can physically handle the flash. I have two other requirements that I am sure will eliminate almost, if not everyone. 1) I will not pay another dollar to anyone until a successful, working prototype is provided. 2) The finished product is for my personal use as an individual photographer, so I cannot just hire a firm to attempt a solution for large sums of money. Just hoping that somewhere out there, someone might find this problem interesting to solve.

I can be contacted via my website: http://BruceVarner.com/ Thanks again for everyone's input and help!

PS. Please do not suggest that I just hold the shutter open, trigger the flash, and close the shutter. If that is what I wanted to do, I would not have traveled down this road in the first place. Also, please do not suggest any of the existing solutions out there for "similar" flash issues, such as Wein Safe-Sync. I have tried them all, and they do not work in this type of application. I even called engineers at Wein & Paramount. They all told me that there solutions were designed by applications with old electronic flashes, not flash bulb, and their products do not work for my situation.

Thanks!