Bear in mind that AFAIK log pots often are not really log - more like two
or three linear sections with different resistance per degree.

On Sat, Nov 28, 2015 at 6:44 PM, Bob Blick <bobblick@ftml.net> wrote:

> I have faked log taper pots with a linear pot and a resistor. If you
> have a 1 meg pot as a voltage divider, strap a 100k resistor from bottom
> (CCW) to wiper.
>
> You could try the same thing in reverse, make a log pot more linear. In
> this case tie the resistor from top (CW) to wiper.
>
> Play around with the resistor value and see if it gets you close to
> where you want to be. You can still use a lookup table, but maybe it'll
> give you a little better resolution.
>
> Cheerful regards, Bob
>
> On Sat, Nov 28, 2015, at 01:28 PM, Denny Esterline wrote:
> > It was/is a project with a budget vanishingly close to $0, living with =
a
> > LOG pot is probably preferable to spending $1 on this.
> > Junk box parts, beefy IGBT, leftover power supply, junkyard automotive
> > ignition coil =3D big fat sparks for the kids to play with.
> > Pots are controlling frequency and duty cycle of the IGBT.
> > Logic is provided by $3 arduino nano clone. (Not a fanboy, but $3 is
> > tough
> > to ignore)
> >
> > LOG pot works, but the area of interest is about 20 degrees of knob
> > travel.
> > Good enough for the purpose at hand, but it's one of those things my mi=
nd
> > chews on.
> >
> > This morning I created an inverse LOG lookup table based on
> > y=3D147.6*ln(AD_reading), where AD_reading is 0 to 1023 and 147.6 was a
> > number I played with till the output values also scaled to 0 to 1023.
> > Yeah, 1k lookup table is a bit excessive, but simple and as John
> > mentioned,
> > fast. Neither are too critical, as this entire program is about 10 line=
s
> > of
> > C (well, arduino brand C flavored code substitute)
> >
> > Based on completely subjective feel, the area of interest is now ~150
> > degrees but it does not appear that the center of travel produces
> > mid-range
> > values. I'm guessing this is the reality that this is not an actual LOG
> > taper, but a piece-wise linear LOG approximation. In a more critical
> > application, I could see measuring the physical position of the pot whi=
le
> > reading the A/D value and creating a better lookup table. More work tha=
n
> > this is worth I think.
> >
> >
> > -Denny
>
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