Adding an expression pedal jack to add resistance to existing pot

[mvdkwast]

Hi ! Newcomer here, and English is not my native language so I hope you'll get what I mean.

I have a project that has a pot, and to which I would like to add a jack input so I can add an external expression pedal. The pedal should not override the pot, but rather add its resistance to the value of the existing pot, so I can set it to a minimum level, and boost it with the pedal. To illustrate what I mean, I made a crude drawing.



The upper part shows how the circuit should behave if no jack is plugged in. The lower part shows the case when a jack is plugged. My problem is that I have a hard time figuring out which jack to choose, and how to connect it. If found a link with many jack types, but it's kind of overwhelming.

Thanks for any pointers !

(oh, and the question is not about the resistances' values here, it's the signal voltage is used through an ADC)

[GibsonGM]

Have you used a simulator, such as LT SPice, to try out different arrangements?  You are on the right track!  And I would think any SHORTING-TYPE T,R,S jack would work fine.   Of course, you need the jack to be a simple short circuit when nothing is plugged in.  So it is just a matter of the mechanical connections.

What I would look at is how the two pots could interact (if both were set to zero, or maximum).  If you need a definite minimum level, you would have to hard-wire a resistor in there.   Otherwise, you are certainly coming close to what it seems you need! 

[Mark Hammer]

I would be very careful about this, depending on what the original pot is supposed to do within the circuit.  Since your expression-pedal cable will be carrying audio signal, and not just a "control" signal, that cable may result in adding noise that could be amplified by another part of the circuit.

It there a way that you could use the expression pedal to control something else?  For example, could the "extra resistance" be a photocell, and the expression pedal adjusts the brightness of an LED?

[mvdkwast]

@GibsonGM: Interaction is not a problem. The worst that happens is that the range of the pedal gets smaller as the pot resistance increases, but that's not really an issue here. The idea was for instance to tune in some delay with the pot, and to be able to increase the amount by pressing the pedal. I'm planning to use old sewing-machine pedals that return to 0 resistance when released in addition to an expression pedal that stays in place.

@Mark Hammer: No in fact it will be a control voltage, noise will not be a problem. I was considering posting this in the digital fx section, but as this is more an analog question, and my construction is a happy mix of analog and digital, I chose to ask here.

[mvdkwast]

Replying to myself for posterity. A solution with a switching jack.



My local dealer doesn't have much, and I couldn't find the neutrik NMJ5* on the internet, so I ordered a NMJ6x instead, which also has a switch on the sleeve.