Or even better the PC Sync port - it's quite a simple plug, easy to get one and connect to the camera, the hot shoe is a bit fiddly to connect to (unless you have a spare shoe and are willing to modify it).
Also setting flash sync to the second curtain will close the circuit just before the exposure ends instead of when it starts, which may be beneficial for interval calculations with long exposures.
I've tried the flash port with rear curtain syncing, but still didn't do what I needed it to do. Mainly, I had difficulty with the timing of the signal. It's not like the shutter release where there's voltage and then there's not. It's an obvious thing to test for. The flash was just a quick blip. Maybe it was the low cost meter that I have, but when the flash triggered, it barely even registered on my meter.
Yes, it should be a short blip. It's just closing the circuit for a brief moment to allow the discharge of power from a capacitor into the bulb. Hard to detect by a meter, but should be easy to pick up by an Arduino or something like that, maybe even a simple circuit.
A (cheapish) multimeter is going to be too slow to catch the signal from a flash shoe, however any cheap microcontroller or electric circuit should catch the signal with ease.
So, any suggestion on what the voltage from that port would be. That was my hang up. Since the multimeter wouldn't register, I was just guessing. I started at 5v similar to the shutter release, but never could get a reading from it to register on my Arduino.
There's no voltage. Maybe your meter could register drop in resistance (from ~∞ to ~0). Think of it as simple push button. Or, to put it differently, you need to supply your own voltage and it will flow for a moment when the camera triggers the flash (closes the circuit), and you can register the spike.
Edit: the circuit is insulated from the camera electronics since some flash lights can send 250V or even more trough the PC socket - it acts as the pathway for the charge from capacitors to the bulb (in some, esp. older strobes).
