So what we have here today is a PT2399 delay, the base of which I lifted directly from madbean's Sea Urchin (with a few filtering value changes to shave off hiss in the delay line). I added an LDR in parallel with the Time pot and a resistor in series with both of those (to set the minimum time). The LED that modulates the LDR is activated by a momentary switch (normally open). The LED on/off speed (or rise/fall time) is controlled by some limiting resistance and a large cap. The rise speed is fixed to a greater degree, but the fall time is user-controllable through the Decay pot. This little part of the circuit could certainly be improved upon, but it works as is.
So how does it work / what does it do?
The LDR is in parallel with the Time pot. The LDR's dark resistance is about 5M, and in parallel with 25K, that gives ~24.9K, so the LDR is effectively not in the circuit when dark. Pressing the switch lights the LED, which causes the LDR's resistance to fall to about 500-ish ohms. This brings the Time pot value down to about 490 ohms (but there is a series resistor of 10K that keeps the value between pin 6 and ground from going that low). If this change happens really fast, there's not much interesting to hear. But if you slow down that change, you get some cool sounds. Especially as the LDR goes back dark and the Time pot value increases back up to 25K (plus that 10K in series). It's similar to slowly turning the Time pot manually.
It's like a cool detune that goes up in pitch and then slowly returns to the root note. And you can control how quickly (or slowly) that transition happens. You can do crazy tape wow-and-flutter sounds, and also more subtle bends. And you can do it all with your foot instead of bending over to fiddle with the Time knob. You can also quick tap the momentary to get multiple pitch bends in a row. I'm still figuring out stuff with this nutty thing.
I really HATE hiss in a delay, and the 2399 can be prone to it at delay times greater than around 200ms. Check out C14 on my schematic. It's part of a three-way low pass filter (pins 12, 13, and 14), which is also the first filter in a second-order low-pass. I found that in order to reasonably kill the hiss, the actual cutoff frequency for the first filter is 33hz! That doesn't make sense because I know that I can't hear anything below about 100hz (I've tested my hearing). But that's what the math says. And I know that I can go more extreme with the filter and it makes an audible difference.
I think changing up the first LPF is loading the second one in a good way. I'm too lazy to do the math on all that.
Anyway, here's a so-so demo video:
Some build pics:
And finally the goods:
