So I'm trying to modify my passive volume pedal to be a volume pedal / USB (arduino teensy) expression pedal. So it would still function as it normally does with the 250k(audio) part of the dual-ganged pot, or if the USB (arduino) is plugged in, it would function as a MIDI-over-USB expression pedal.
Initially I thought about getting a dual-ganged B10k / A250k so each gang would serve its ideal purpose, the A250k for volume and the B10k for the arduino/expression.
I've had a hard time finding a pot that is A250k/B10k, much less that is a compact frame with a long shaft, like what will be needed for the mounting:
An analogue input has no problems reading a 250k pot.
But because of the higher impedance, hum/hash pickup could be a problem.
If the wires are long (>20cm), use shielded (audio) wire.
Two consecutive analogue readings, and only using the second one, could also be needed for stable readings.
"Log" might be your biggest problem.
You can try to make readings of that 250k log pot more lineair by enabling the internal pullup resistor of the analogue pin by adding.
pinMode(A0, INPUT_PULLUP); in void setup()
Assuming A0 is the potPin.
Leo..
It seems the OP wants to retain his existing 250k log pot to control the analog volume, and another coupled linear pot to be read from the arduino for other purposes... am I right?
You can plot the resistance values as you swing the pot from 0 to 250K, say getting 10 values as close to uniformly spaced as possible. Now you can linearize by first mapping the 10 values you got to a linear ramp up from 0 to 250 K. Now google "piecewise linear" and you will see how to create a correction in software for your logarithmic input.
allanhurst:
It seems the OP wants to retain his existing 250k log pot to control the analog volume, and another coupled linear pot to be read from the arduino for other purposes... am I right?
I understand OP wants to replace his single 250k log pot with a dual gang 250k log pot.
Because he can't get a double 250k log and 10k lin.
jrdoner:
You can plot the resistance values as you swing the pot from 0 to 250K, say getting 10 values as close to uniformly spaced as possible. Now you can linearize by first mapping the 10 values you got to a linear ramp up from 0 to 250 K. Now google "piecewise linear" and you will see how to create a correction in software for your logarithmic input.
A pullup resistor, as explained, can be used to pull a log pot towards lin behaviour.
Not ideal, but probably usable.
Leo..
allanhurst:
It seems the OP wants to retain his existing 250k log pot to control the analog volume, and another coupled linear pot to be read from the arduino for other purposes... am I right?
Allan
Yeah that's correct.
I did think of a lookup table as well, but worried that in the extreme ranges that it would get steppy or inconsistent, which wouldn't be ideal for the purposes of a pedal controlled expression thing.
There are the "mod pots" too, which I hadn't ever heard about:
I submitted for a quote, but since it's only one, I can see that being quite expensive.
Is there a place that just sets the components piecemeal? I don't need especially weird values (250k audio taper and 10k linear).
Oh, I kind of have a feeling that "audio taper" is "reverse log" is that right? Or is it regular log?
kriista:
I did think of a lookup table as well, but worried that in the extreme ranges that it would get steppy or inconsistent, which wouldn't be ideal for the purposes of a pedal controlled expression thing.
If the 10-bit analog inputs on the Arduino are not good enough, you could use one of these inexpensive 16-bit external adaptors:
So wasn't able to order a modpot as one of the elements was out of stock and they'd have to order them in in bulk.
So I've been looking at more DIY options, like buying a dual-ganged 10k/10k that uses conductive plastic elements, and kind of cracking it open and sliding in an A250k into one of them.
This looks nice and compact, but I would worry about the stability of the body of this as it would be in a foot pedal, so there would need to be some structural strength.
And they appear to make them in a couple of different resistances. It looks like the bottom just unscrews too.
allanhurst:
You're using the pot sections as ratio dividers - so the actual resistance values aren't that important - have you checked what values are available?
For the Arduino/digital part that makes sense, but wouldn't it need to be an actual A250k if being used as a purely passive pot?
For the Arduino/digital part that makes sense, but wouldn't it need to be an actual A250k if being used as a purely passive pot?
Nope - just like the linear pot - given that drive source impedance is low enough and the output sink impedance high enough, the actual nominal value doesn't matter.
