# Theatre special effects: LED hand flash



## Carl Foxmarten

For a local theatre company I'm building a few special-effects props, including a palm-mounted (likely on a glove) unit that produces a bright flash of light, enough to momentarily overpower the stage lighting.

I've picked up a 980 lumen LED that runs at 3 amps and 3-4.2 volts, and I'm wondering if I need anything more complicated for switching the LED than a high-enough power transistor?

Others I've spoken with have only been experienced with constant-lighting applications, so recommended switchable LED drivers, which I haven't found yet. Any suggestions?
(though not strictly necessary, it'll have a microcontroller for precise timing of the flash. Though a 555 timer might be more prudent if the LED takes too much current)


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## Carl Foxmarten

Um, anybody? Anyone at all?

I'm open to switching to another LED, if somebody can explain how to design the control circuit. I'm just unclear on the control circuit side of the project.


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## TEEJ

What about a flash driver like from a camera flash?

All the camera phones use them now, just dissect an old one.

Many flashlights have a strobe function, one flash of that might be all you need...and you can borrow a driver from them too, etc.



What you might consider is what the lux the theater lights are shining into the audience, and how many degree wide area is that covering.....so you know what lux you need your light to flash out at them....to overpower it.

You'd want to make sure the lux is adequate....so start there, and work backwards to a light that can do it.


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## Carl Foxmarten

Our last magic wand has a xenon flash unit from an old film camera in its tip, along with a white LED to show that it's ready to flash again. The only problem with this is that it needs a 4" diameter ball on the end to house all the parts. The problem with using something like this for a hand-mounted flash is that you're running some _very_ high voltages through wires from the palm of a glove, through the actor's clothing to a small box on their hip. We're talking about somewhere between three- and five-hundred volts, with the jolt that starts the electrons flowing through the xenon gas hitting ten times that, at up to about five thousand, so it _really_ isn't very safe at all.
(I'm still trying to locate high-voltage wire for this application, if I can't figure out how to replace the flash unit with an LED that is...)

Flash units that use LEDs are _only_ found in cellphones, as they're the only place where xenon tubes don't fit. Unfortunately, the LED flash units from actual phones I found on Amazon don't go much higher than a few hundred lumens (not sure what that comes out to in lux), so I'd likely have to cobble it together myself, somehow. Which goes into rather advanced territory for me, as I have absolutely no experience doing such things, and haven't found many, if any, useful resources for building a circuit based off the datasheet of an unfamiliar type of chip...

Another issue I have is that I don't actually have any hard numbers. At all. The fairy (who uses the wand, which currently uses a flashtube) is typically doing magic in more brightly-lit scenes, and the demon (who would use the hand-flash if we had one) would generally be using it in the darker scenes. But how much light is getting thrown is very much a mystery to me at the moment.

Oh, and did I forget to mention it's for panto, as opposed to other forms of theatre?... =>.>=


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## PhotonWrangler

Does it have to flash once during the scene or more than once? I'm wondering if a supercapacitor can be used to dump it's charge into the LED on command (with a zener diode across it to protect from spikes). This would only work for a single flash though and would need time to recharge.


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## Carl Foxmarten

I'd rather give them the option of having multiple flashes per scene just in case it's needed.

Would I need a supercap, or do you think I could get away with a regular electrolytic?
I'll be experimenting with a "normal" high-brightness white LED later today.


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## PhotonWrangler

Carl Foxmarten said:


> I'd rather give them the option of having multiple flashes per scene just in case it's needed.
> 
> Would I need a supercap, or do you think I could get away with a regular electrolytic?
> I'll be experimenting with a "normal" high-brightness white LED later today.



I have my doubts that an average electrolytic can store enough energy to produce a good flash but go ahead and try it.


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## Carl Foxmarten

Hmm. In that case, where would I buy a supercap from then?
Preferably online, as there are very few stores in my area that go terribly far beyond hobbyist-level electronics...


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## PhotonWrangler

Digi-Key or Mouser Electronics would be good places to find them. Look for a 1 farad cap.


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## Carl Foxmarten

And since my current LED is rated for 3V, is that the voltage I should go for as well?
(if I used a resistor to limit the current, I'd be essentially wasting some of the power 

Digi-Key, at least, only has 5V supercaps in a 1F capacity, and if I were going for 3V it'd be somewhere between 4F and 50F. Sounds like a bit too much to me, but then I don't have any experience to work from here. Similarly, Mouser has 3V supercaps from between 4F and 50F.


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## SemiMan

If I was doing this:

- I would likely create a "wand tip" with a bunch of XPG2 or XML, etc. on a circuit board crammed as tight as I can make it. Perhaps 4-6 LEDs, all wired in series. Then I would put 4 of these around the end of the wand-tip, and wire them in parallel, i.e. 16-24 units total in a 6S4P configuration. I would use a boost circuit to charge up suitable capacitor (regular may be fine) to say 24V, maybe a little higher. I would charge it through a resistor so that I had a suitable charge time (1-10 seconds?). I would put a momentary switch between the capacitor and LEDs with a small value resistor to limit the maximum current to about 20 amps. I.e. 24-(6*3.5)/20 = 3/20 = 0.15R though you may find you don't even need this as the LEDs can take quite a hit short term (i.e. to discharge the caps). The caps also have internal resistance, as does the switch, wires, etc.

If you use XML2, that will give you for a brief moment about 25000 lumens. Not as much as a Xenon strobe, but if you get long enough duration, i.e. 20-50 milliseconds, it will seem just as bright.

Semiman


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## SemiMan

On second thought, for ease of design, I would likely use two small A23 alkaline batteries in series for 24v and replace them every few shows worst case.

In terms of the LEDs you may be able to just glue them and hand solder wires. Heat sinking will not be needed for short duration pulses.


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## PhotonWrangler

+1 for Semiman's well thought out solution.


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