I built Dean Hazelwanter’s Holtek HT8950 Pitch Shifter / Robot Voice / Vibrato from the build notes at General Guitar Gadgets.
For fun, I used an optoisolator (VTL5C3) to control the 250K freq pot (P3).
I connected the LED leads of the optoisolator to the LED pads on the PCB, and the LDR leads of the optoisolator to the pads for P3.
Here’s what it sounded like. . .