using a single pot in multiple circuits?

[alikins]

I've been trying to figure out the best way to multiplex a single pot to work in
multiple circuits.

For a more concrete example, I'm currently contemplating
taking the guts for 6 danelectro octave pedals and shoving them in a
container and running input from a roland gk pickup into it. With the
idea being to have a polyphonic octave (similar in principal to the
boss oc-20).

But I would like to have just one pot control the equilvent settings
for all six circuits. The only solution I can think of is a single led/
multiple ldr optocouple type setup.  Is there a simpler approach?

[Dutchie]

maby not simpler is a approach where u use a pic ( or other microcontroller ) to steer digital controlled resistors.

or use fet's as a voltage controlled resistor solution

Greetz

[ExpAnonColin]

You could also use H11F3 photofets if you got the voltages down right... not that complicated at all.

Or a 6-ganged pot, right?

-Colin

[niftydog]

RG will tell you that digital potentiometers are mostly useless for applications over 1mA.

I would think your best bet is to use a pot to drive a transistor/FET array.  The LED/LDR combo is gonna be hard to get consistant accross the 6 circuits.

If they are all identical circuits, and the circuit is such that you could replace the pot with a variable DC voltage that controls the effect... perhaps a small voltage, variable DC supply connected in parallel to the wiper terminal on the PCB.  Then all pedals would have to have a common ground and +V supply.

[alikins]

I would think your best bet is to use a pot to drive a transistor/FET array.  

If they are all identical circuits, and the circuit is such that you could replace the pot with a variable DC voltage that controls the effect... perhaps a small voltage, variable DC supply connected in parallel to the wiper terminal on the PCB.  Then all pedals would have to have a common ground and +V supply.

Is there a good example of driving a fet array like that somewhere I could
take a look at?

I need to take a closer look at the circuit and see if I can use the second idea.

[alikins]

Or a 6-ganged pot, right?

-Colin

You know, that occurred to me but I didn't even get it any serious though
figuring there was no such thing. A bit of googling seems to have proved
me wrong though:  http://www.potentiometers.com/partdata.cfm?partno=580X&session=2004060706453436

Don't know how expensive those are, but that would certainly be easy and flexible. Might
need an odd looking case to fit those in though ;-> (course, if I'm putting six boards in
the case, thats probabaly not avoidable...)

[niftydog]

Is there a good example of driving a fet array like that somewhere I could
take a look at?

not that I'm immediately aware of... but someone else here might know.  there has been discussion previously on this board about similar topics, perhaps try searching the forum? sorry!

[R.G.]

I'd try using a switched capacitor technique to fake the pots. That's got the potential (sorry...) to give you a semi-infinite number of highly matched pots as needed.

Not only that, the parts are cheap (caps and CMOS switches) and easily available. Have you priced one of those 6-gang conductive plastic jobbies?

Some circuit redesign may be needed, depending on whether the pots are used as variable resistors or voltage dividers. Linears are much easier to fake with switched capacitors than log pots.

[yano]

you could use multiple gang pots...but i don't know if they come in more than dual gang

[Athin]

R.G. - how does the switch cap technique work ? any schematics or explanation available?? waz this already discussed [where - I can't find it!]. I wanna get a couple of schems from runoffgroove and connect them to a a-class tube power amp and don't want to have 15 knobs on the amp.

[petemoore]

Take a couple dual ganged pots, point the shafts at each other, but about 1/4'' apart, make a mounting plate [U shape] to mount them in that position, when inserting the shafts, use a clear plastic or other tube [with an ID that's tight over the shaft, so as to drive it] and slide the shafts into the short tube ...
 

  Shaft-----tube------shaft
 Figure out a way to rotate the tube, 4 pots get driven at once.
 I used the Lo-Fi, just a plastic gear wheel, with a melted in king pin through the tube and attached to the wheel, then strips of duct tape, I'm sure you could figure out something neater...tricky part is boxing it neatly...I cut a slot from center of top of box, to end of top [where side starts] and down the side 1/2 way, and mounted the drive wheel so it protruded out of the top and side a little bit.
 The other way would be chain or belt drive and would require some machining for it to be neat, or even functional.

[Paul Perry (Frostwave)]

I don't know abt 'switched cap' but, i'vew had trouble wit 'switched resistors'. Because, it is difficult to get the 'on' time narrow enough for a high resistor value, using simple ckts (triangle & comparator).
plus, you might need a low pass filter after everything to filter out the ultrasonic switching freq.
But, i can see using it for one of those low pass filters that have a 4 gang pot! maybe controlled by a PIC, that would make it easier.