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)
>=20
> 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 mind
> chews on.
>=20
> 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 lines
> of
> C (well, arduino brand C flavored code substitute)
>=20
> 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 while
> reading the A/D value and creating a better lookup table. More work than
> this is worth I think.
>=20
>=20
> -Denny

--=20
http://www.fastmail.com - Access all of your messages and folders
                          wherever you are

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.