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Tuesday, October 14, 2014

Beomaster 6000 4-Channel (2702): LED Based Scale Illumination (Pt.1)

Something different today: While I am waiting for a set of Beocord 5000 belts from Martin (Dillen on Beoworld.org; thanks much for providing all these great replacement parts! - much appreciated), I thought it was a good time to report about my efforts to convert the scale lights of my Beomaster 6000 (2702) to LEDs. I have to admit that I am a bit of a LED fan, and that I never appreciated small incandescent light bulbs. They just break too easily, especially during transport, and I think if the designers in the 70s had had shorter wavelength LEDs at their disposal they would never have used incandescents. I am aware that there is a discussion out there about whether to converting to LEDs is somewhat sacrilegious, but this is a free country, as they say...;-):

Here we go: When I opened the Beomaster 6000 up for the first time several of the light bulbs illuminating the scales on the controls and the FM dial were broken. Worse, several of the small plastic bulb sockets had broken out tabs as seen here:


The only way to fix them in their proper places would have been with tape...not something I am into...Beolover only likes neat solutions!

So, my first response was trying to reproduce the sockets, but I could not find anything resembling the metal brackets that are inside the original sockets. Then I considered using LEDs, and I thought that amber colored LEDs would give a pretty good interpretation of incandescent light. So I designed same-formfactor replacements for the lamp sockets, which would hold an LED and a current limiting resistor. This worked really nice with the Makerbot II using its highest resolution settings and 50% infill. Here is a pic of the result:

Pretty! The LEDs were Dremeled into cylinders to make them emit light in a non-directional diffuse fashion. Here is a pic of the setup in comparison with the original incandescents:

I enthusiastically put them in place. Here at the bottom end of the control scales (the color appears a bit too orange in these pics...in real, it really looked quite like small incandescents):

And at the top:

Pretty neat, I thought. Here is a pic of the FM dial with LEDs installed:

So far so cool...unfortunately, after installing the controls and the FM dial back in place, when I put the dark plexiglass cover of the Beomaster 6000 on, I had my first Waterloo moment with this project:
I turned the light in my lab off, and observed the Amber light coming from the scales, and first I was happy, it all looked very authentically incandescent, but then it occurred to me that the red lines on the indicator bands were hardly visible.
At this point it dawned on me that there is a significant difference between incandescent light that looks amber and amber LED light.

I used CREE C503B-AAS-CY0B0251 amber LEDs with an emission wavelength of 596 nm. Here is the emission spectrum from the data sheet:

One notes the narrowness of the amber emission peak, which hardly overlaps with the peak of their red LEDs, and herein lies the problem. The red indicators look red because they reflect only red light (or in other words, they absorb all photons that are not red), hence if one throws amber photons at the red indicators, they will simply absorb those photons and no light is coming back from them.
That the indicator looks red in the photo of the FM dial is a result of the ambient incandescent light in the room that was present when I took the picture. I forgot to take a picture of the vanishing indicators when I observed it...was just too disappointed. They were really almost as black as the rest of the bands...

This meant, I needed to find some red photons! Initially I thought to use white LEDs instead. White LEDs usually down-convert blue LEDs via a phosphor layer, which produces a certain amount of red photons (hence the white looks). Here is a spectrum from the white LED I used, made by Multicomp (MCL053SWC-YH1):

Note the strong blue light peak...that comes from the LED, and then the fairly broad, but much weaker incandescent-like down-conversion 'hump'. The problem with this is that the blue part of the spectrum is very strong, i.e. this is much more like sunlight than a small incandescent light bulb. Here is a spectrum of a typical incandescent light bulb (from here):

Note the weak overlap with the visible spectrum, and the practical absence of blue light. Mostly IR radiation, i.e. heat and very little visible light. This suggests that the scales would look way to white, while the red indicators would be reflecting red light at a much weaker level. Here is an impression from the very white light coming from these LEDs (of course I had to try it despite knowing better at this point, but hope springs eternal...;-):

It became clear at that point that the only way to replace the lightbulbs with LEDs would be with RGB LEDs, or at least with red-green types (which are available). The only problem with these is that two resistors are needed to control red and green independently to be able to select the right incandescent sheen, while providing enough red for a solid reflection on the indicators. I realized that there would not be enough space for discrete resistors of the proper wattage in my printed sockets.
So, my solution was to use WS2812B digitally controllable RGB LEDs with built-in control electronic. I always wanted to build something with them, and here was an opportunity. Here is a pic of these cool devices (from Adafruit):

They are just 5 mm square, i.e. fit perfectly into the cavities for the light bulbs. One can chain them up with just three leads and then address them like a shift register. This enables awesome multicolored tens or hundreds of LED ribbons that are able to display fast colored patterns. My application is of course more quaint with only six resp. four units, but more about that in my next blog entry.

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