Friday, June 7, 2013

...and fricken lasers!

I keep hoping to get started on that machine gun.  Or, for that matter, the drum magazines.  But the hot projects on the table now are lighting effects for the next production.  So my work table is covered in light pipe, LED's, Arduino clones, and laser diodes.

Like these:

The first task is to put some kind of glowing wires on a costume.  EL wire is neat, but just not bright enough for stage lights.  I'm also concerned about the RF noise/whine from the inverter.

LED strips are difficult to defuse.  The next thing I'll try is surgical tubing, but so far for any smaller "wire" diameter it looks too much like individual blobs of light.

Light pipe seems the best option so far.  SparkFun has the stuff; 3.5mm or 6mm flexible plastic tube.   You stick a light in one end and it is piped down the tube.  The most useful for this application is the "white core" stuff, which diffuses the light as it goes, meaning the length of the tube shines.  The downside is that light pipe isn't smooth, either; the intensity drops off steadily down the length of the tube.  But this is an acceptable look for what we are trying on stage.

The first thing I reached for to illuminate the pipe was "Piranha" LEDs.  This ubiquitous package is the same as used in the BlinkM.  They are typically 20ma per channel, giving the RGB Piranha a total output of about 60ma.

The big advantage in the RGB is, well, being able to select color.  And by adding an AVR, I also have the option to flicker, pulse, chase, or otherwise dynamically change color and intensity.  To demo this, I cut a 30cm length of light pipe and superglued a Piranha at either end.  I plugged them both into an Arduino I had lying around, and (after a brief but worrying search for where I might have archived the source code) re-purposed my custom BlinkM software to show off a couple of possible looks.

It was colorful, and of course controllable, and it was just bright enough to show up on stage in less-bright lighting cues.  Given that it changes over time, this may just work as a subtle accent on the costume.

But more power would be good.

I tried a couple of blue LEDs, both a 20ma 5mm and a 40ma 10mm one.  The 20ma was actually almost as bright; because the beam spread on that was a mere 20 degrees, more of the emitted light got into the light pipe.

And if a narrow beam angle is good, then the natural thing to do was hook a 20ma green laser into it.  Which was nice and bright!  With my 100ma green laser diode, the stuff was visible in direct sunlight.  Not that I'd ever put that on stage.  Not without very good ways of protecting cast and audience from any accidental spills of laser light.

Which led to the one 3W "Cree" type LED I had lying around.  Also an RGB.  I hooked the green channel of the Cree to a 4.5V battery pack with a one-watt 100-ohm resistor as ballast.  (I also tried it direct on 3V, trusting the internal resistance of the AA batteries to limit the current.  The star-shaped aluminium heatsink became warm to the touch, and I abandoned that experiment after a few minutes!)

In any case, the Cree outshines my 40ma laser.  Which seems natural, as the thing gobbles down 350ma on each die.

So I've put a handful of Cree on order.  And also lens assemblies that will both channel more of the light (the die has a natural 180 degree view angle) and make it, I hope, easier to connect it firmly to the light pipe.  

I'm also considering constant-current power supplies.  DX has them for quite cheap, and the First Imperials have been using them with good result in their blasters.  On resistor alone, the LED ran without getting hot for a good four hours.  But I think I can get a little more output.  Plus; the "MR16" constant-current drivers are based on PT4115 chips, and thus include a PWM input.  That saves from having to include power darlingtons to switch the high-power LEDs from the AVR.

And was that whole paragraph above just completely greek?

Which brings us back to the picture at the top; EL wire with mini-inverter, my Arduino-powered Piranha, and on the 4xAA battery pack, one Cree.  All draped over my little scroll saw.  I need to clean the workshop again, I see.
That's in daytime, by the way, with room lights on.  Both the Cree and the ends of the Piranha Pipe are blowing out the CCD, but because of human eye versus CCD sensitivities, in the real world the glow of the EL is barely visible, and the Cree feels about twice as bright as the Piranha.  Which, given the power law of human visual perception, works out pretty close (80ma on one, 350 on the other).

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