LED/LDR VCF?

[Matt Haines]

>I have schematics of LED/LDR based lp vcf at
>
>  http://www.uta.fi/~helin/sch/ledldrlp.gif
>
>That circuit has no cv-converter, but I'g going to use inverting
>summing amp (like Electronotes VCF-option 1) -> NPN -> four PNP-
>transistors with 470 ohm resistor limiting the current.
>The 2k feedback resistor must be changed to much bigger, as well
>that 150k from 2N3904 to V- smaller. How much current does a LED stand?
>
>Has anybody ever tried led's and ldr's in vcf? Craig Anderton's
>book Electronic projects for musicians contains several
>circuits that use Clairex's CLM6000 optoisolator, are they
>available these days, or are there other some kind of devices?
>
>Don't tell me this is is a stupid idea because it is. The filter will
>not be linear, poles are not placed at the same frequencies, and
>it's gonna be very slow. CLM6000's "attack" time is 10ms and
>"release" time much, much longer (about 100ms).
>
>Are there any other reasons not to give a try to it? I'd like to know
>if it oscillates or not and how does it sound, maybe that's a good reason
>why to eventually build it?  Next time I'm going to use motorized
>potentiometers or thermistors+infrared transmitters, that would NOT
>be cool.

Hey, this is very cool, and just what I needed! I was looking to make a
4-pole LPF that was simple, cheap, non-CV (i.e. just a manual pot to
control it) and speed and accuracy were not crucial. Looks like you just
handed it to me!

As for reasons not to try it...heck, try it! I'm going to. This type of
LED/LDR set-up was often used in envelope followers aka "auto-wah". The
LDR's slowness is actually a benefit in this usage, because you get
smoothing of the waveform for free. I.e. an audio signal is input and
full-wave-rectified, and then converted to current to drive the LED. The
LED is flashing on and off like mad, but the LDR is responding only in a
low-pass manner to it, so all the bumps of the individual waveforms are
smoothed out.

If you need a bunch of filters for something, this would be very useful as
it's cheap and simple. You could throw in another LED/LDR and have
VC-resonance too! BTW, the Mutron III and a DOD auto-wah work using a
similar set-up as your schematic, except the Mutron at least has band- and
hi-pass filters as well (and isn't 4-pole). ElectroNotes envelope followers
don't work like this since they're not integrated with the filter, and thus
use an attack-decay set-up to simulate the LDR's tardiness.

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Matt Haines  haines at apc.net         .       .       .       .
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Aliens suck.