Electronics > Projects, Designs, and Technical Stuff
Ultra Short, Ultra Fast LED Flash
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JAndrew:

--- Quote from: Zero999 on March 06, 2019, 10:16:29 am ---I've done this before. The requirement was for a 1µs. Phosphor LEDs are variable. Some are fast and others are slow.

Attached is a schematic of the test set-up I used to drive LEDs with 1µs pulses of hundreds of amps, hence the low value current limiting resistor. It's possible to overdrive LEDs by a factor of 10, for short pulse lengths. Note transformers are used to drive the MOSFET and measure the current, to avoid large currents passing through the grounds of the test equipment.


--- End quote ---

I'm working on digesting all the idea's that have been suggested here. Zero999, I'm just making sure my flea brain is understanding the diagram,  you use an inductor to drive the LEDs rather then using a Flash Capacitor.

I appreciate the lead on the LED you were testing. Vela mentions they spent a lot of time trying to find the right LED's for the job. I suspect this gets back to the issue of how long the phosphor stays excited. I'm fairly confident they are using white LED's, the images of the unit show the LED's and the yellowish tint characteristic of white LED's is clearly visible.  This would simplify the circuit, as we wouldn't need to worry about white balancing the RGB LED's. Since LED's MFG's aren't typically concerned about the switching time of a White LED, I can image there is a lot of variation, possibly even from lot to lot.


--- Quote from: Marco on March 10, 2019, 03:49:56 pm ---
--- Quote from: JAndrew on March 10, 2019, 01:24:42 am ---I've studied his schematics and everything he's doing makes sense. I am assuming the length of the pulse is being controlled by the software on the MC.

--- End quote ---

Microcontroller control only really makes sense if the micro triggers both the chain of events you want to photograph as well as the flash.

If you are say pulling the trigger on a gun with a string it makes much more sense to just keep everything analogue and trigger on sound or a light gate, no micro necessary.

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As far as triggers go, I was planning on using an IR optical sensor. I've put one together before, and have had fairly good success with it. As the bullet passes by the "shadow" creates a signal. The trouble with using this directly is that the duration of this pulse is dependent on bullet velocity and bullet length. Fast short bullets would create pulses that would likely be far less then 500ns, while long slow bullets would create a pulse that would last far longer then 500ns. (These type of sensors are commonly used in high end chronographs)

So we would want the MC to trigger on the pulse, with a programmable delay (just a few microseconds) before sending the trigger for the flash.  By having a short delay, we can dial in the position of the bullet in the frame without physically moving the set up. I know I've tried to use an Ardiuno to create a short 500ns square wave but I seem to remember that it struggled. I don't think the code was fast enough to switch the I/O pin that quickly.  It will be another problem I'll need to work through.


To Marco's comment of using low voltage and high amps, I was thinking on that last night. A 10s (37v) Lipo battery rated at 4000mAh could provide a pretty good jolt of energy across an LED. With the duration being so low, I don't think there would be any lasting damage to the Lipo and it would be good for quite some time. There may be other reasons that this wouldn't work, but I was brainstorming.
ogden:

--- Quote from: Marco on March 10, 2019, 03:49:56 pm ---
--- Quote from: JAndrew on March 10, 2019, 01:24:42 am ---I've studied his schematics and everything he's doing makes sense. I am assuming the length of the pulse is being controlled by the software on the MC.

--- End quote ---

Microcontroller control only really makes sense if the micro triggers both the chain of events you want to photograph as well as the flash.

--- End quote ---

Right. Attiny and software definitely is not up-to task for external trigger application. It possibly could be done in *hardware* of other MCU - like advanced timer of stm32 clocked at 72MHz. Using such one can make adjustable delay and pulse length with 14ns error. If programmable delay is not needed then just pulse length has 14ns error.
Sceadwian:
Yeah I missed some of those earlier posts thanks, some good info there. I've noticed the phosphor afterglow myself apparently it's normal. The action/reaction of phosphors has never been anything I"ve looked into, get into chemistry and some atomic physics that have a bit more nuance than I care to get into :) Thanks for the graphs though! Very helpful.


--- Quote from: Zero999 on March 10, 2019, 05:12:21 pm ---
--- Quote from: Sceadwian on March 10, 2019, 04:12:02 pm ---Is this even possible with white LEDs? I thought the phosphor delay was WAY more than the nanosecond range?

--- End quote ---
It depends on how fast and the LED. I posted an oscillogram of the CMA3090 being pulsed at 1µs here. As you can see there some delay in the on and off time, but not much. As I said in my previous post, some phosphor LEDs are worse, others are better.

What's odd about phosphor based LEDs is, although the initial decay is fairly fast (even the slow yellow one I tested had a time constant in the µs range) a very faint afterglow persists for a long time after the LED is turned off. This is no problem for a photoflash application, because it's several orders of magnitude too small to cause a problem: it's just about detectable, with dark adapted eyes, in a very dark room. I've seen it for myself. I ruled out persistence of vision and charged capacitors by closing my eyes, exposing the LED to a backlight (UVA) whilst my eyes were still closed, turning off the UV source and looking at the faint afterglow from the LED. I have only done this with white LEDs and don't have the equipment to make quantitive tests, otherwise I would.

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Marco:

--- Quote from: JAndrew on March 10, 2019, 06:20:47 pm ---So we would want the MC to trigger on the pulse, with a programmable delay (just a few microseconds) before sending the trigger for the flash.

--- End quote ---
Do an analogue delay, a current source charges up a capacitor, you compare it against an adjustable threshold with a fast comparator.

--- Quote ---I was thinking on that last night. A 10s (37v) Lipo battery rated at 4000mAh could provide a pretty good jolt of energy across an LED.
--- End quote ---
Nope.

The amount of energy you're burning is tiny, so you just use electrolytic capacitors (they actually have pretty low inductance). It's advantageous to keep the powersupply which charges those capacitors low current, because if something does go wrong by any chance that limits the continuous current which can flow so everything can't blow up. Unlike what happens if you short that LiPo stack.

None of this is entirely trivial, it's quite a complex project. I think I'd design it with a comparator on the input with adjustable threshold and feedback to the input with a diode (so once triggered it stays on, until you arm it again with a physical button, add a signal LED so you can see if it's armed). This will turn your trigger signal into a step signal. The step signal goes into your pulse generator with adjustable delay and pulse duration, a fairly complex circuit ... too complex to describe in text. That then goes to the drivers/LEDs/MOSFETs. You need 2 boost converters. One boost converter for the MOSFET drivers, one boost converter to charge the flash capacitors. Also one linear regulator for the trigger section.

All in all, a lot of components.
JAndrew:

--- Quote ---None of this is entirely trivial, it's quite a complex project.
--- End quote ---

I definitely see that. I've learned quite a bit in the last 48-72hrs just going through everyone's responses. Honestly its likely a little above my current level of understanding. If I am going to make this work I'm going to have to spend some time on smaller and slower circuits. Particularly looking at/recreating the examples of what has already been done.

I appreciate everyone's input, I definitely have a better sense of where to start, and how to eventually get there.

To be clear, you can still take some pretty awesome pictures with flash duration of 2-4 microseconds. I just set a target, based on the specs of the equipment I had used prior.

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