[sdiy] Performance control - processing a control voltage

[Matthew Smith]

Harry, Tom, many thanks for your input on this.

The main thing that has come out of this, both from your comments and my 
muddling around on paper, is that I'm not going to get it out of a 
single 5V supply, at least not without having an on-board boost 
regulator and then creating a virtual ground. [My original design, from 
which this is an offshoot, uses a digital output sensor - going analogue 
to make the whole thing more "organic."]

One of my design criteria for just about everything is that it should 
run off a wall-wart, so that whatever plugs into the mains can be fully 
compliant with local regulations, and the apparatus as a whole not 
require certfication. (I just ain't going there.)

So - I'll see if I can get my hands on a 24V wall-wart (possibly an 
after-market laptop power supply) and use a rail-splitter and buffer 
(it's a Burr Brown part, can't remember the number, ditto the rail 
splitter) to give a virtual ground. If that turns out to be unavailable 
or too expensive, I'll probably use a closed-frame MeanWell Supply that 
gives me +12V/-12V/+5V straight up. Element14 (Farnell) carries such as 
these, and I can generally screw them down on price pretty well.

Tom - what's the basis of 14 x lowest resistance - is this a hard and 
fast rule (of thumb)? Just for information, the LDRs I have go from (and 
I'll have to test and trim them individually) 8k-24k light resistance to 
at least half a Meg dark resistance.

But Tom's schematic wraps up what I really couldn't figure out, which 
was do the offset first, or do the gain first - or do come magic and get 
them in a single stage.

I'll have a fiddle based on this schematic, and actually nut out the 
details experimentally, in hardware.

---- Now for the boring bit ----

What this is actually doing.

The unit I'm designing is a multi-sensory sculpture; in essence, a synth 
and a light show in a single (Perspex) box.

Light source is a set of RGB LEDs, mounted in various positions in the 
box.  A set of mirrors, some moving (stepper motors) and some static, 
combined with some surplus optics from Electronic Goldmine make up the 
visual side. Synth bits are hidden underneath. The LDRs are (tastefully) 
mounted in various locations within the box, so get illumination from 
the LEDs, or reflected light thereof. CVs go to a patch panel which then 
routes to the synth controls and also to the ADC inputs of a 
microcontroller which drives the steppers and the LEDs. It's not a 
totally closed loop, as ambient light will also affect the inputs.  The 
result should be chaotic - a constantly evolving pattern of lights and 
tones.

Well, that's the idea, anyway!

Now I've just got to figure out how to make a mirror drum entirely out 
of Perspex.

-- 
Matthew Smith

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