I built a very similar "lightning trigger" based on a photo transistor about 5 years ago, and unfortunately, I found the idea does not work.  At least not for a DSLR.

A DSLR requires ~100ms delay to take a picture after being triggered.  This is the time it takes for the mirror to flip and open the shutter.  By this time, the lightning strike has mostly ended.  Mirrorless cameras can have a delay well under 10ms.

Most successful lightning photography involves just opening the shutter and hoping you capture a strike.  You might try using this idea to open the shutter and then using the lightning detector to close the shutter after you detect a strike.

I built something like this many years ago to capture things like light bulbs breaking. It was a film camera, so the idea was to shut the lights out, open the shutter, and smash the bulb with a hammer. The sound would trigger the flash, making your exposure. DSLR makes this so much more versatile. Nice job!

Thanks Dan!! That's pretty much the process I've used to capture objects falling into water with my DSLR. I might need to find some old bulbs to try.