I am not sure you need to tap into the circuit because you have the LED, just use a photo-sensor, an OP-AMP and then fine tune the gain to get a logic high (or low) when the LED is fully lit. Just glue the photo-sensor onto the LED with blackened material (black hot glue)kaleun96 wrote: My hope was to tap directly into what powers the flash recycle indicator LED but as you see in the photos linked above it's a minuscule SMD component that would be a nightmare to wire onto. With time, patience, and a oscilloscope I'm sure I could find a better place to tap into it but in the mean time I'm going with an externally mounted log-scale light detector to pick up the LED brightness. A bit tricky in of itself and less robust but it should work I think.
I do not recommend differential circuit though, because the LED might light up slowly for some brands, thus making it difficult to detect the first order derivative for all types of flashes.
But as far as I remember, the LED is not very accurate, sometimes, output energy can vary by half stop, even with some high end flashes, like Nikon SB900, etc. My Canon 580EX, that particular unit, has about 1/3 stop variation.
Just my 2 cents