Sorry about that.
When I take a picture very close , it is difficult to focus because all is very dark. Fo fix this problem, I made a small pannel with 3 LED and this is good enough to focus. Currently the LED are allways on and I can use the flash.
The light from the flash is enough to override the power of the LED light, but sometimes the LED create reflection, and to avoid this I am trying to turn off the LED light when the flash is activated.
In practice the LED light should turn off when the flash is activated, and return in the on position after.
I could have a simpler solution using the sync cable between the camera and the flash, that is actually just an on/off switch, but I would prefer a totaly independant solution if possible.
I hope i explained the process.
Now that I understand what you are trying to do maybe myself or the others can help you. I have been following the thread more out of curiosity. It motivated me to try and toss a circuit together. I was looking at assorted pieces and parts lying around in hands reach so I threw something together with what I had. Keep in mind these are odd ball parts.
I had an old photo cell lying here (actually a few) and was curious if the photo cell could react fast to a flash. Especially with a crude circuit. I used a LM339 comparator (actually 1/4 of a LM339) and a 7474 Dual D Flip Flop. Those parts plus a few others and it actually works.
A drawing is attached as well as a picture of the rats nest bread board. Using a single comparator of the 4 in the LM339 I set a reference voltage on pin 5 using a 50K potentiometer. I would have rather used 100K but I had a few 50 K pots available. Keep in mind I used what I had. I also used a 5 Volt supply as it was convenient. This could be done with lower power chips and lower voltage chips. I set my reference voltage on pin 5 of the LM339 at about 2.5 Volts. I then adjusted my pot in series with the photo cell so it wouldn't trigger the comparator with ambient room light.
The output of the LM339 on pin 2 drives a 7474 type D Flip Flop. Normally at power up the RED LED is illuminated. When a flash fires the flip flop does its thing and the GREEN LED lights. I simply used two LEDs for test purposes. For your application you would replace the single RED LED with a transistor to drive your several LEDs. The reset button in the circuit will "ARM" the 7474 and the RED LED will again illuminate.
Notice in the picture of the rats nest the GREEN LED is illuminated. Every time I tried a flash and took a picture the red extinguished and green came on showing only the green being illuminated. Meaning that if there was no green connected the red would be extinguished when the camera actually caught the image.
Since flash can vary I tried this with two cameras. I used an older Canon EOS 10D and a new Canon EOS 7D for test purposes. The results were pretty much always the same. I was actually surprised the photocell responded as well as it did.
I am not saying this is an ideal solution but figure it could be a good start. I am sure someone here can design a similar circuit using newer parts. Additionally I figure this could all be done on a single little 8 pin micro controller chip. I didn't take that route as many people don't want to mess with programming a uC chip.
Anyway, hope this helps a little...
<EDIT> I did not show decoupling caps on the chips power pins. I would use .1 uF caps there. I did not show the remaining 3 comparators in the LM339 which would have their inputs tied to ground. Although I wired both D flip Flops in the 7474 I only used the outputs of one of them. Again, this was just a rough draft of a test circuit.
</EDIT>
Boo, Boo I just noticed in the drawing that Pins 2, 4, 10, and 12 were not tied to 5 Volts and they should be or no workie, workie. My bad.
Ron