On/off modulation circuit?

[Luke51411]

I'm messing around with some pt2399 delays and I added a momentary switch that places a resistor in parallel with the time pot. I like the effect but I'm wondering what would be a good circuit to create the same on/off effect only automatically. Would this be considered a square wave?

[mth5044]

You could feed a transistors' base via a current limiting resistor a square wave LFO. Wire the transistor switch in parallel with the delay pot (collector to one side, emitter to the other) and have your other resistor in parallel. When the transistor is open, it will bypass the resistor, when the transistor is closed, the resistor will be in effect. Could work?

[samhay]

You can easily generate a square wave from a relaxation oscillator - the common single op-amp LFO - by tapping the non-inverting input.

However, I am not sure how muscial this would be, so I would consider making the effect more versatile but adjusting the LFO shape/depth so you can adjust from no modulation, through some 'wobbling' to your extreme on/off effect.
A number of PT2399 designs use a transistor or LDR in series/parallel to pin 6 of PT2399 to do just this (or most of it anyway).

[Luke51411]

It would probably have limited use. The way I have the momentary switch set up right now is with a 100k resistor across a 50k pot. It creates a pitch drop on activation and raises the pitch upon release then goes back to normal. It almost sounds like 3 separate notes are played and also slightly like R2D2 or some other generic robot. I think it might sound cool in a continuous mode. I think the added resistance is high enough above the pot value that it doesn't create an unpleasant sound as the resistance drop is not That much but just enough to create some interesting pitch shifts and delay modulation.

[samhay]

What you are describing is the modulation feature found in a bunch of PT2399 delays like the Echo Base and Zero Point delays.
It is also how my Cherub chorus works.