How does a DSLR trigger a manual flash unit

If you want to detect a shutter actuation or build a DIY wireless flash trigger, it's not as straightforward as it seems, there is a thing you would need to have in mind.

A week ago, I got my first manual flash (from late 80's). I was worried about putting it on my DSLR, because on few places I read that the so-called "trigger voltages" of some old flashes are 250V which would certainly damage the sensitive electronics in my DSLR. So, I measured the voltage that the flash unit gives between the CENTER PIN and the pin on the side of the hot-shoe mount (GROUND PIN). The voltage was just over 12V, I decided to risk it and put the flash onto the DSLR. And... It worked perfectly fine!

Having a manual flash with just two pins, made me wonder what would it take to trigger the flash with (let's say) Arduino Nano remotely from the camera. From an online schematic, I learned that the flash gets triggered by connecting the CENTER PIN to the GROUND PIN. That easy! It is essentially completing the circuit on one of the transformers side, so the other can light the lamp very brightly. I guessed that there is a transistor in the DSLR that simply connects these pins together.

NOTE: Online I found other schematics which suggest very high voltages through the CENTER PIN, which could be bad for ordinary low voltage low amperage transistors. (That's if you want to put a transistor in between).

Going further, I wanted to learn is this all there is to it (when the camera shutter goes down, the flash is triggered for few milliseconds and than it should go back to normal). But what I discovered is a little bit different...

Here's how I hooked up an LED with a simple 3V battery supply that was acting as a charged flash unit through R1 (47Ohm) directly giving power to the CENTER PIN.

I inserted the modifies hot-shoe on my Nikon D5200 and actuated the shutter once. As predicted the LED turned off, but I guessed just after few milliseconds it will come on again (just like a quick pulse). That never happened. The LED didn't come on until I restarted my power supply. Here's a video of the whole action: https://www.youtube.com/watch?v=Z2YFKX_Ie7E

So what this tells us, is that the camera is pulling through the flash's trigger voltage until the flash is empty (in our case with the battery acting as a flash, until I turn the battery off, the camera is still going to pull the voltage to the ground and not give any to the LED) Then it stops, and of course that flash's going to recharge again. 

If you want to see the LED flash, you would need to use transistors to restart the battery voltage. I don't have currently time for creating a schematic, but it's going to be fairly easy. I just wanted to learn what the camera does and in the future maybe build a DIY wireless flash trigger.

Some other day, I may extend this. It was just a simple learning project for today.

- Mitko