Saturday, May 26, 2012

Blink..and you miss it

I thought about titling this "Don't blink" in honor of the great Doctor Who episode, but that would sound like I was recommending against the ThingM products.

And that isn't true.  I think the BlinkM is, unfortunately, underpowered for theatrical use.  I took my breadboard out to the theater and viewed it from stage distance against a variety of subdued lighting looks and the best that can be said is that it will light up a diffuser globe real well with color.  Casting light you can see on an actor, though...not so much.

So it is time to play Tool-Tim Tim, but there are a bewildering variety of options for "More Power!"  I spent a while trying to spreadsheet them (and downloading lots and lots of datasheets from different manufacturers).  And, well...for a smaller and simpler battery-powered device the BlinkM MaxM is actually quite competitive.  So my work in hacking up alternate software wasn't a complete waste (although I really need to rewrite the core function as an interrupt-driven loop -- and there are some cute PWM tricks you can do to speed things up even more).

At the lowest price end of the options, the ATtiny chips can sink 40 ma from each of the I/O pins.  Each channel in an RGB "Piranha" LED is about 20 ma.  So technically one CPU chip could drive as many as three Pirahna, making it 3x the power output.  Which of course will not appear 3x as bright in most situations!  There are other 20 ma LEDs around, but most of the super-bright and ultra-bright (which inevitably give misleading specs in millicandella) are narrow-angle.  And that just isn't appropriate for most of the lighting applications I have in mind.  One exception is the "flat-top" LED sold by some Hong Kong suppliers, which offers 25 ma and a half angle of 120 degrees.

For making a pure candle (or perhaps some RGB thing that is around the same effective output) it is hard to beat the price point of buying ATtiny's and Piranha LEDs in bulk.  Wire them up with no driver, no power regulator...nothing but ballast resistors.  You can make them on a perf or in "dead bug" style for a nice small form factor.   Actually, you can make them brighter and more efficient by sacrificing the RGB and using amber, yellow, or "warm white" LEDs instead.  The main advantage to using multiple LEDs in a candle is that you can move the point source around a little and produce a nice flicker in angle as well as in intensity.

For practically anything else I can think of in theater -- and that includes a railway lantern or any period portable light sources other than candles -- we need more power.  Which introduces drivers to the equation.  The simplest driver circuit is a Darlington.  The handy ULN2803 can sink 8 channels worth at 500 ma each from a basic 20-pin DIP package.  The venerable TIP120 power Darlington can swing as much as 5 WATTS.

Wiring up multiple LEDs yourself can be time consuming.  SparkFun carries the "Satellite" from the same people who built the ShiftBright; it is a grid of 13 single-color Piranha meaning it puts out about 300 ma per color channel.  Which makes it about 15 x the lumens of the BlinkM.  Adding the driver and CPU and some discretes, however, brings you close to the $24 of the MaxM -- which is already assembled, and uses just three high-power RGBs to put out the same juice in a more compact package.

So to make it worthwhile you need to jump up to the Luxeons and similar LEDs pulling from half an amp to a whopping 10 amps each.  SparkFun does have breakouts, but they are pricey; it brings the surface-mount Luxeon Rebel up to about $7 a channel -- not including controller -- and $18 for an RGB array.  Plus you might just want to add diffusing lens, additional heat sink, even a constant-current source.

At which point you step back and say, "I'm not trying to do UL-listed architectural lighting here; I just want an effect for stage use."  And this makes some of the Hong Kong suppliers hanging out on eBay very attractive; as some of them offer 1-3 watt (300 to 750 ma) single-color surface-mount LEDs with rudimentary breakout boards/heatsinks for as low as a buck each.  Throw in some TIP120's to switch them, hack out some primitive PWM code on an ATtiny, and if you are lucky it won't burn out (or worse, burn up) before the show closes.

Which, if someone comes up to me today and says, "We want Willy Wonka's cane to light up when he taps it on the floor," is what I will do.

And I'm still going to stick my modified BlinkM in one of those cheap plastic candlesticks to show it off.

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