LED/LDR VCF?

[Christopher List]

> 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.

Another neat use for this is as a combination is as a combination low-pass 
filter/analog delay. You put the LED at one end of a black tube and the LDR at 
the other - for every 186,000 miles you add to the length of the tube you get 
another second of delay!

- CList