Before I got started I looked at this wikipedia article about lightning (Lightning - Wikipedia) so that I could verify this project would work. It has a lot of interesting information about lightning, but the most useful piece of data in the wikipedia article is the time lapse shot of a lightning strike. From the time lapse photo I was able to determine the duration of a lightning strike is about 100 ms. Then from this page (ImpulseAdventure - Shutter Lag & Startup Time Comparison) I found my Canon 30d camera has a shutter lag of 65 ms. I know from a past project that if I use a reverse biased photo transistor to detect light it has a response time under 1 ms. The last piece of delay is the software running on the Arduino board and since it's running at 16 MHz I am sure I can run a tight loop that takes under 1 ms. Adding up all the delays, I get 67 ms which is still much less than the 100 ms duration of a lightning strike so I was pretty confident this would work before I started work on the prototype.
The image sequence here File:Lightnings sequence 1.jpg - Wikipedia shows 8 frames taken over 0.32 seconds. That works out at 40mS per frame. I would have thought you only had two frames here that would trigger your unit, that's 80mS, very close to your delay.
It depends on how soon you will be able to detect the bolt as it grows. This means that the trigger level is a vital aspect.
How about connecting a pot to one of the analogue pins and using that to set the threshold. In that way the next time you have a storm you can easily adjust the level without having to download to the Arduino all the time.
Anyway prove me wrong and post a picture after you find a storm.
I still haven't had a good lightning storm :(, but I have a feeling my current method will work. If it doesn't I'll connect up a laptop and quick change it to the threshold method and see if that works. One of the reasons I think it will work is because the 65ms shutter delay from Canon is a conservative measurement.
I look forward to getting some great pictures and sharing them with you folks. I just need a lightning storm
i'm actually considering building something very similar, but using a TSL230R light to frequency converter... sadly, however, i'm in atlanta and rain is rather scarce at the moment... though, the sensor itself is very sensitive and dealing with a number that directly represents a light level is easier to interpret... i am simply polling directly in the sketch loop, which is as fast as the arduino can process... that said, i've really no way to test against lightning at the moment (but i missed a storm last night!!)... maybe i'll just put it in the dark and turn lights on/off quickly... or maybe fire a flash in the room?...
