Electronics > Projects, Designs, and Technical Stuff
Ultra Short, Ultra Fast LED Flash
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Wolfram:
It probably makes sense to get an idea of exactly how much light you need, this is one of the most important design specifications and it affects a lot of design choices. A few exposures using the planned camera system, and a charged electrolytic dumped into an LED through a resistor, will already tell you if your planned 75 W for 500 ns is a realistic starting point.
JAndrew:
When I approached the project. I broke it down into two main problems. The trigger, and the flash.

The trigger is fairly easy to solve, there are various optical sensors I can use to trigger the flash as the bullet passes buy. Even put in a variable time delay.


The flash and the flash duration has been the harder problem for me. Again my thinking was using power MOSFETs to control a bank of LED’s. If I needed more light I would add more banks of LED but the circuitry would all be the same. Regardless how I go, my thought would be as long as I have one circuit that works setting up several in parallel or looking at ways to overdrive the LED’s for a brief moment wouldn’t be a challenge.

I tried to use an Arduino as a pulse generator. I figure that would be a pretty easy way to go. I could specify the length of the flash, time delay, and even do several flashes. However I don’t think I could ever get the program to run fast enough to switch the I/O pin that quickly. I remember playing with it, and looking at things like the Beagle Board or Raspberry Pi to see if I could do with with a faster processor. Even bought my first O-scope for this project.

I suspect you’re right a 75w target is not enough. Perhaps even the number of LED’s needed to get to the right level of light are impractical. So it could be I’m going about this wrong. Perhaps I should look into what makes a “General Radio GR GenRad 1538-A Strobotac” tick. (I’m thinking aloud)

I haven’t looked spec sheets for all of the different chips that have been suggested or studied everyone’s response. I’ve been tied up at work. I’m going it over this weekend so I suspect I’ll have a few questions.

Thanks for all the input.




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Marco:
I suspect than LEDs are 10-100x more efficient than sparkgaps though, so the difference might not be that large.
StillTrying:

--- Quote from: JAndrew on March 09, 2019, 03:57:44 pm ---Perhaps I should look into what makes a “General Radio GR GenRad 1538-A Strobotac” tick. (I’m thinking aloud)
--- End quote ---

It's a smallish 800v tube, 0.5us to 5us, I've not found a matching data sheet.
Triggering is easy compared with getting the light level up!
Wolfram:

--- Quote from: Marco on March 09, 2019, 04:46:21 pm ---I suspect than LEDs are 10-100x more efficient than sparkgaps though, so the difference might not be that large.

--- End quote ---

It's hard to find good sources for the luminous efficacy of an air spark, but it seems to be in the range of a lumen-second per joule. A non-overdriven LED will therefore be roughly hundred times more efficient, but 75 W is also about five orders of magnitude below the peak power of a normal air gap flash. The total light output for an LED driven at 75 W for 500 us will therefore be about a thousandth of that of an air-gap flash like the EG&G Microflash 549.
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