Is there any Multi Turn LOG/AUDIO pots?? If not, how can I make one with a LIN pot?

Hey all..
I'm working on something where I need a highly precise Potentiometer but it
has to be LOG. I want to use a multiple turn pot (for the resolution), but I
can't find any that are listed as LOG, only LINEAR? Does such a thing
exist?

I want the Potentimeter to be 100K.. and according to a few sites I can make
it close to LOG by adding a 35K resistor.

But, on the "beginners guide to pots", it shows a graph:
http://sound.westhost.com/pots.htm

and it ends up being more S shaped than log, but lower down, it shows a dual
ganged volume balance control using a linear pot and the resistor, and the
curve definately looks more log to me.

Mike
http://mikebeauchamp.com

 

Just to clarify more, because I seemed to have confused myself looking at
thsoe diagrams since they were in a Voltage Divider situation..

My situation is this.. between two contacts, I need to vary the resistance
in in a Log function, compared to the Linear turning of the potentiometer.
Just hooking one edge of the pot up to one contact and the other contact to
the wiper of a LOG pot would be absolutely perfect for me. BUT, regular log
potentiometers don't seem to offere the resolution and precision that I
need..

Mike
http://mikebeauchamp.com

 

How about using a gear reduction drive with a standard 270° log pot?

--
We now return you to our normally scheduled programming.

Take a look at this! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

 

It would still be linear, but with a different slope.

Possibly a large off center pulley where the rate would change depending on
the position of rotation similar to some of the exercise machines
(nautilus)?

Louis--
*********************************************
Remove the two fish in address to respond

 

This solution would take a higher degree of ability to implement, but is
spot on, and could result in a very precise result depending on the care in
its implementation. Another positive aspect of this method would be the
ability to tune the output for any anomalies in the surrounding process.

Louis--
*********************************************
Remove the two fish in address to respond

 

Louis, I stated that you need a log pot, not a linear one. Also, you
should put some mechanical stops like a notched wheel and a heavy steel
pin or block to prevent damaging the pot. The gear reduction drive will
have a lot more toque than a straight shaft.

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida

 

If I needed that degree of control, I'd skip the pot and ADC and use a
shaft encoder to set the address into the lookup table. Could also
have a digital readout and would not have any discontinuities (pots
get noisy eventually). The stability would be limited to the
performance of the voltage ref into the DAC.
GG

 

---
Excellent idea!

Plus, if there's no need for an analog output he could make the output
of the lookup table whatever he wants and be done with the DAC and
reference.

 

Louis, with a sentence like that you should consider going into politics!

niftydog

 

I need a highly precise Potentiometer but it has to be LOG.

Yup. I've got one of these DIY-shunts on my Grandpa's old portable radio.
The results are useable. It has an exponential response.
What the exact curve is isn't critical.

You'll notice that the referenced page is about AUDIO.
What's that they say about good enough for rock'n'roll?