Originally posted by Ex Finn. Post your results when you get done with the build. I am curious about the delay times of the shutter activation and your home brew circuit.
You are hard-core, taking the route of transistors and not an op-amp.
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Not a question of hard core or not.
Look at the requirements. We have to achieve a short circuit, or a low impedance any way, between two pins in the camera shutter release circuit.
The easiest way to achieve this is simply using a transistor.
By also adding a photo diode between collector and base, and using the available power from the shutter release pin of the circuit, any light that turns on the photo diode will bias the transistor and trip the shutter.
All you need to make this a reliable lightning trigger is to adjust the gain of the circuit by adding a variable resistor between the base and emitter of the transistor, which will shunt away some of the photodiode current, and therefore offer an adjustment for ambient light. the circuit is really really simple, and requires no power source,
Why should I use an op amp which needs power, when I don't need to?
As for lightning and reaction times, there have been many many published articles about this, and many lightning strokes last quite a long time, and are initiated by development of an ionized channel with stepped leaders, that work their way between the clouds and earth in 25-50 foot lengths, once the channel is formed there is then the full strike and usually a reverse strike or several restrikes in the same channel lasting some time.
the ~100mS reaction of a DSLR is fast enough for this. No need for bulb mode, and note bulb mode is hard on camera batteries, and also leads to heating of the sensor because it is powered for so long.
As I said, the concept works very well using a simple $15 flash slave, but it does not have the sensitivity necessary for lightning.
I may revisit this as I already have a slave to camera converter built. It is just a question of potentially modifying the slave unit.
Edit Note.
I have out of curiosity, taken the cover off my flash slave unit, and discovered, no surprise here, that the circuit is just as I have described, one transistor, one photo diode, and one resistor.
It would seem simple enough to replace the resistor with a potentiometer, to make the sensitivty adjustable.