Replacing pots with stuff controlled by microcontroller?

[ytsestef]

Yeah, well, sorry about the somewhat confusing title, hopely I'll describe better in here.

I got my hands on Arduino (http://arduino.cc) and got really excited. I want to experiment on using it to control some gear. Like replacing all of the controls and maybe some VU meters. Stuff like:

- Having a single tactile button that switches between compression ratios on a GSSL/1176, lighting up the corresponding LED (Using Relays)
- Having some LED arrays for Gain Reduction or Input/Output levels
- Relay true bypass (ok, no need for Arduino here, but makes stuff even easier)
- In a farfetched scenario, even replacing all the controls with digital ones using an LCD with menus and cursor buttons and stuff (just because it is possible)
- Adding Store/Recall preset functions

I can probably do all that stuff if I try hard enough, however I have one basic question. What is the best way to replace pots?
I have done a small internet search and found out there are several solutions. Some are less accurate, some are expensive, some are tricky.
I have so far read some stuff about FETs (using resistors to linearize them?), digital pots (?), some people are even using optocouplers to do such stuff and motors to control motorized pots (obviously do not want this!)

So what do you guys think? Using CV to replace pots (resistors essentially) using a microcontroller (input could still be a pot, or a rotary encoder or even buttons).

PS: I do realize that it gets even trickier if we want to emulate log pots.

 

[sahib]

............... and motors to control motorized pots (obviously do not want this!)

Why not. I am already working on a similar project. I am designing my own servo controlled potentiometers though. By the way programming is done by a close friend of mine who lives in Athens.

 

[ytsestef]

............... and motors to control motorized pots (obviously do not want this!)

Why not. I am already working on a similar project. I am designing my own servo controlled potentiometers though. By the way programming is done by a close friend of mine who lives in Athens.

The reason not to do this IMHO is that it is much more expensive and sophisticated than necessary and it leaves me with the impression that it will have limited accuracy. What are the odds of the motor stopping at the exact same spot when recalling a parameter compared to the odds of the microcontroller delivering the exact same voltage to the some FET?

Nevertheless, I am looking forward to see your project complete and in action!

 

[mrclunk]

I've been looking into doing this also. I had grand plans for a digitally controlled Sontec eq mastering beast.  But i've almost given up on the idea.

I was mostly looking into emulating pots (not attenuators) using relays. Controlled with microproccesor, arduino seemed pretty cool and theres a good community of enthusiasts out there.
Basically i found out to emulate a single log pot accurately u would need 8 relays.
A stereo sontec has 10 x dual rev log pots for starters so thats 160 relays just for the frequency controls!

Salhib i'm interested in your motorised pots tho.

some links
http://www.groupdiy.com/index.php?topic=655.0
http://stiftsbogtrykkeriet.dk/~mcs/
http://jos.vaneijndhoven.net/switchr/index.html
http://www.ozoneasylum.com/31285

 

[abbey road d enfer]

http://www.maxim-ic.com/products/digi_pot/

 

[ytsestef]

Thanks everyone for your answers and links.

Mrclunk, 8 relays for a massive gain knob on a hi-end preamp with no other controls should be really good, but for eq/comps and stuff with multiple controls, it gets way out of hand.

The digi pot seems to me a bit overcomplicated (although I'll admit I'm a newbie), and also seems to need a fair amount of arduino outputs. It is a possibility, though, if everything else fails 

http://freespace.virgin.net/ljmayes.mal/comp/vcr.htm

What do you think of that? Seems like linear is possible, but could it be made to behave logarithmically?

 

[sahib]

............... and motors to control motorized pots (obviously do not want this!)

Why not. I am already working on a similar project. I am designing my own servo controlled potentiometers though. By the way programming is done by a close friend of mine who lives in Athens.

The reason not to do this IMHO is that it is much more expensive and sophisticated than necessary and it leaves me with the impression that it will have limited accuracy. What are the odds of the motor stopping at the exact same spot when recalling a parameter compared to the odds of the microcontroller delivering the exact same voltage to the some FET?

Nevertheless, I am looking forward to see your project complete and in action!

Yes it will be more expensive comparing to a solid state component or a circuit. But for DIY this should not be an issue unless you have commercial intentions.  In terms of control it is not sophisticated at all. Also integration into an existing analogue design is much easier. Besides, you do not have to worry about the additional noise may be introduced by the solid state modification. What literally you have is all digital control in the front with pure analogue in the signal path.

For the motor to stop at the exact spot on recall refers to repeatability, and even the cheapest standard R/C servomotor will give you excellent results. I design low cost robot arms and manipulators driven by RC servos and even that sort of  heavy (at the low end) application metal gear versions maintain a good accuracy. But this is not a worry for driving a potentiometer.

However, they normally operate upto a maximum of 180 degrees. By removing the stopper lug on the output gear you can drive them up to 270 degrees, which is the mechanical limit of the feedback potentiometer inside. This gives you a resolution of 1.05 degrees in 8 bits. In terms of electrical resolution I do not think you would need anything more than that in audio.

Obviously coupling the servo to the potentiometer is the mechanical obstacle that you have to overcome. You can couple them side by side but that becomes exteremely bulky and impractical if your potentiometers are sitting close to each other on the front panel. That is why I am designing my own. I am at a very early stage just now and it will probably take me until summer before I can release anything tangible here on the forum.

 

[stickjam]

Unless I'm misinterpreting the data sheets, the challenge I've seen with digital potentiometer chips is pot voltage in relation to signal/supply voltage.

A lot of them use like a +3v single Vdd supply and the data sheet says that the voltage on any of the three pot pins must remain within the range between Vdd and Vss (ground).  That seems to rule out using them in any typical analog scenario that might have something like +/-15V rails. 

I hope someone with more digital pot experience can correct my above interpretation of that spec which, for example can be found in this data sheet in the Recommended DC Operating Conditions table on Page 7 under "Resistor Inputs".

 

[ytsestef]

Well, this wouldn't really be a problem, since most microcontrollers use +3.3V to / +5V, anyway, including arduino's Atmel chip. The basic problem is that I don't know much about "digi-pots", including how to hook them up. From a schematic I had stumbled upon, it seems to include a couple of resistors and three or four decoupling caps, which at first seems like a lot of work just to emulate a pot. Maybe that's why it is not too well-known? People seem to go with voltage controlled resistors of all kinds...

 

[sahib]

Unless I'm misinterpreting the data sheets, the challenge I've seen with digital potentiometer chips is pot voltage in relation to signal/supply voltage.

A lot of them use like a +3v single Vdd supply and the data sheet says that the voltage on any of the three pot pins must remain within the range between Vdd and Vss (ground).  That seems to rule out using them in any typical analog scenario that might have something like +/-15V rails.  

I hope someone with more digital pot experience can correct my above interpretation of that spec which, for example can be found in this data sheet in the Recommended DC Operating Conditions table on Page 7 under "Resistor Inputs".

Read this application note.
http://pdfserv.maxim-ic.com/en/an/AN161.pdf

 

[abbey road d enfer]

AD5291/92 operate at +/- 16V on the analog side, 2.7 to 5.5 on the digital side.

 

[abbey road d enfer]

Read this application note.
http://pdfserv.maxim-ic.com/en/an/AN161.pdf

I think there are much simpler ways of handling the problem. Anybody with experience on working on single-supply circuits would find their solution a tad "ethereal".

 

[PRR]

>> +3v single Vdd supply .... voltage on any of the three pot pins must remain within the range between Vdd and Vss (ground). 
> Well, this wouldn't really be a problem, since most microcontrollers use +3.3V

You don't need a pot to control your computer (anymore).

The pot controls the audio. Most consoles and limiters have +/-10 Volt signal swings. A Fender Bassman has nearly 100V swings on its tone controls.

Re-engineering the audio so that NO point swings outside 0V-5V is a major problem. And there may be some reason the original designer went to the expense of +/-15V or +24V rails. (Although sometimes only for external drive; you could do the tricky-bit at 5V then use an X4 buffer to the output.)

So we have the chip "pots", mostly 3V-5V though some 30V. Mostly these are "potentiometers" which won't pass wiper-current without distortion or error, ruling out the many rheostat applications for knob-"pots". Most aim at Volume Control, and may even have a common "ground" for two pots, which is handy for hi-fi but some other stuff likes floating pots.

There are motorized pots of several flavors. The aircraft servos are fast and accurate with good power/cost choices, and some microcontrollers have model-servo subroutines. You can also get motorized knob-pots from old home hi-fi gear. These are true pots (can be rheostats), stereo (even 5-ch) with decent tracking. A particular point about motor-pots is they HAVE a Readout and they HAVE a memory and they have manual over-ride: when your micro goes brain-dead they tend to come-up in the last-used setting, you can see the setting on the knob, and (unless the motor-drag is bad) you can use digital techniques (the digits on your hand) to turn the knob. Yeah, I know Aruindo is the greatest thing since sliced dodo, but _I_ don't trust them to never fail. ('Specially if I programmed it....)

The photoresistor is another possibility. They truly-float and there's no real voltage limit, altho THD rises above a few volts. They are NOT pots, you may need two plus some trickery to set each one along a non-linear curve. This curve varies from part to part, so has to be calibrated, or fed-back using a dual-part where the two units may be more-equal than two singles from the same bin. But mostly if you contemplate conversion from normal pots to photoresistors you want to think about re-designing the whole system around single variable resistors instead of potentiometers.

Much audio manipulation "can" be done with VCAs. Volume pot is usually followed by a gain stage, the two together could be a VCA. All the passive and NFB EQs "can" be re-engineered with VCAs. But costs add up, and some re-design is not trivial. (Try making a Fender tonestack between two hard-worked 12AX7 using dBx VCAs instead of simple pots. Don't forget to emulate the input and output impedances, which affect overdrive tone.)

Rememeber we use pots in ways that are not necessarily "best", just the best we had in 1947. If you want to set the decay rate of multiple meters, PWM may be cheaper and more accurate than trying to servo pots or photo-Rs or even digi-pots. And having a brain in the box ALWAYS leads to new maybe-better possibilities than simply "replacing hands". Why do we set limiter release with a pot? Wouldn't it be nicer to watch the signal, sneak-out a couple dB in each rest, hold in sustained medium passages? That was unlikely in 1936, difficult in 1966, too-easy in 2006.

> their solution a tad "ethereal".

Me, I'd say ugly, goofy. That they'd even publish that "solution" suggests someone isn't understanding the problem.

 

[abbey road d enfer]

Me, I'd say ugly, goofy.

You're such a nice sport... ;D

 

[ytsestef]

>> +3v single Vdd supply .... voltage on any of the three pot pins must remain within the range between Vdd and Vss (ground).  
> Well, this wouldn't really be a problem, since most microcontrollers use +3.3V

You don't need a pot to control your computer (anymore).

I was actually talking about the the computer's output that would feed the [digital] pot.

Anyway, funny thing is, I am now starting to consider motor-pots  ;D ;D yeah, a few hours ago it was me that said "definately not".
What about FETs?

 

[PRR]

> I was actually talking about the the computer's output

Yes, that is not a problem. If you want to stop there, not have any audio, groovy.

> What about FETs?

Poor consistency, and horrible distortion over 100mV signal while half-on. (ON or OFF is not a big problem, part-on is terrible.)

 

[ytsestef]

> I was actually talking about the the computer's output

Yes, that is not a problem. If you want to stop there, not have any audio, groovy.

> What about FETs?

Poor consistency, and horrible distortion over 100mV signal while half-on. (ON or OFF is not a big problem, part-on is terrible.)

ON or OFF could be dealt with relays as well, couldn't it?
So, the only solution seems to be motor-pots?

EDIT: what about this? Stumbled upon it while searching around, haven't heard anything bad about it yet. (Well, it is kind of expensive, but it has two channels)

http://www.ti.com/lit/gpn/pga2310

It seems to be the only one that I am able to understand how to hook up, too.  ;D

 

[abbey road d enfer]

The PGA2310 is configured as a volume control, with a buffered output, referenced to a common voltage. On a GSSL, several of the pots are used as floating rheostats; not compatible.

 

[jdbakker]

http://www.groupdiy.com/index.php?topic=33487.0
http://www.groupdiy.com/index.php?topic=24223.0

http://www.analog.com/en/switchesmultiplexers/analog-switches/products/index.html

http://www.thatcorp.com/datashts/dn130.pdf
http://www.thatcorp.com/datashts/dn120.pdf

and, if you can keep the voltage swing under control and don't mind a bit of distortion (~0.01%), you could consider a multiplying DAC like
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en020399

JDB.