Cheap log pots, according to my measurements, are two linear sections. 0 - R/10 at half rotation and R/10 to R at full rotation. That leads to a very noticeable and disturbing discontinuity around the mid point. I also often wonder why they are called "log" pots when their law is actually exponential? On 29 November 2015 at 18:51, Sean Breheny wrote: > 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 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 botto= m > > (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 wit= h > a > > > LOG pot is probably preferable to spending $1 on this. > > > Junk box parts, beefy IGBT, leftover power supply, junkyard automotiv= e > > > 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 > mind > > > 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 > lines > > > 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 L= OG > > > 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. > > > > > > > > > -Denny > > > > -- > > http://www.fastmail.com - Access all of your messages and folders > > wherever you are > > > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 __________________________________________ David C Brown 43 Bings Road Whaley Bridge High Peak Phone: 01663 733236 Derbyshire eMail: dcb.home@gmail.com SK23 7ND web: www.bings-knowle.co.uk/dcb --=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 .