Idea for digitally controlled analog potentiometers.

[acehobojoe]

Clearly, this has been done before. On various digital soundboards you can recall a preset and it will make the pots slide into place, but what if we developed a way for this to happen with rotary pots that have some sort of robotic gear attached to them? wouldn't that be amazing?

Have all of your analog pedals modded to a singular preset box that automatically dials in your favorite presets.. wired with a 3.5mm cable or something.

[petey twofinger]

as you may well know . neil young used something they called a whizzer setup , for his amps . i seem to remember reading it was fairly complcated ( not digital more mechanical ) , and touchy but , i believe he used that for a loong time / may still use that .

cool idea . i mean having these things controlled digitally .

in the 90's i serviced an older sequential circuits analog synth which had digital presets , but the audio path was analog , that was really cool , i didnt want to give it back , so i did record a few songs with it .

[octfrank]

Motorized rotary POTs already exist, ALPS makes some.

[crane]

I have seen a bass amp with robotic pots (I think it was a GK amp)
BUT - for a DIY project - I would much rather use digital pots and control them via uC.
But then again - it depends on the purpose.

[R.G.]

I retype this every time the idea comes up here.  

1) find small stepper motors with 1/4" shafts on both ends
2) mount the stepper motor to the panel, and then the pot to the back-side shaft with a bit of rubber tubing and an automotive hose clamp
3) use a uC to drive the stepper motor.
The hidden advantages of this are that the stepper makes a slight cogging effect on the knob that feels nice, and the motor turns freely when not energized. It's LOCKED in position when the coils are energized, so the uC can turn it to position, then let it go for manual control.

Clever-er additions are
- use a dual pot, and put 5V across it, then use the uC A-D converter to read the position of the pot
- use the uC to both read and remember pot position, so you can have the overall controller remember where the pot was.

The difficulties are
- mechanics: you have to find and mount suitable steppers and pots
- you have to program uCs and also the overall controller setup.

[anotherjim]

Anyone remember the old Motorola car radios with mechanical presets? You tuned to the station you wanted with the dial then pulled out a button (one of 4 or 5 under the tuning dial) which set a clever friction clutch/escapement thing. Thereafter, pushing that button in drove the tuning dial cord to the preset position -ish.

[electricco]

googlize "arduino tuning guitar"...

you'll find someideas to bring it to the POT

https://www.youtube.com/watch?v=H7QqwZS1cmQ

cheers

[acehobojoe]

I'm going to try to make it happen. Anything is possible.

[amptramp]

There are multiplying DAC's like the DAC0808 that allow you to multiply the incoming analog signal by a digitally set factor that can be set by a µcontroller or an up/down counter.  An additional channel could run a meter which will be your potentiometer position indicator.  Check Figure 9 here for a digitally set amplifier gain or attenuation:

http://www.utdallas.edu/~kamran/dac0808.pdf

The DAC 1208 permits more precision at the cost of greater complexity:

http://www.ti.com/lit/ds/symlink/dac1230.pdf

A surplus store (Globe Electronics) that used to be here (the owner was in his eighties when he sold off his stock and retired) used to stock remote control volume controls as used in some televisions in the late '50's or early '60's but it used a 117 VAC reversible motor so there may be some hum injection when it operates.  Not that much of a drawback for a TV but fatal for a mixer board.

There also used to be some attenuator IC's that were basically FET switches connected to a tap on a resistive divider.  It was set by an internal counter circuit.

[karbomusic]

Anyone remember the old Motorola car radios with mechanical presets? You tuned to the station you wanted with the dial then pulled out a button (one of 4 or 5 under the tuning dial) which set a clever friction clutch/escapement thing. Thereafter, pushing that button in drove the tuning dial cord to the preset position -ish.

I do, they worked pretty well back in the days when any car with a clock, had a clock that didn't work.   

[PRR]

> old Motorola car radios with mechanical presets?

Not just Motorola, though they may have been involved in creating that gizmo.

Bro's 1941 Plymouth has such a tuning contraption; the alternate low-price radio you pulled the buttons and turned a tiny screw behind.

> pushing that button in drove the tuning dial cord to the preset

I've been inside (a lot). No cord. Gears and slip-clutches. Amazing bit of clock-works.



An alternative to the stepper motors is the "servos" used in remote-control planes/cars. A gear-motor with a self-control pot and a chip that compares a pulse-width to the pot wiper, turns to match the percent width/voltage. "Possibly" less brain-work than steppers (though perhaps the same as stepper with stepper-controller). The stepper is very repeatable but does not know where "zero" is (unless you bang it against a stop at start-up like my Chevy does), the RC servo has a mechanical zero. The servo shaft is not 1/4", but the arm adapter has a hub which might be. Many experimenter's CPUs have a servo command.

I think the Wizzer is RC servos. A particular point for Neil is that it sits EXternal to his beloved amplifier, lightly touching the stock knobs, no gut-work. Remember Neil is a fine technician and he has dedicated technicians on staff... without such support it might not get built or might be constant trouble.

But the sweeter trick is to manually set the knobs, press "Store 13", and the CPU *reads* the pot positions to memory, "Recall 13" brings them back. Neither the servo nor the stepper can do that simply. A dual-gang motor pot "can", but the values are limited, and the values we may like for guitar-amp work are generally higher than what a deluxe hi-fi favors or what may be best for the CPU read-back.

> On various digital soundboards you can recall a preset and it will make the pots slide into place

This is how HIGH-end analog mix-consoles worked in the 1980s. "Flying Faders". They were hard to get set-up, costly factory techs on site. They worked really good until the factory came out with an improved system. Sometimes you could retro-fit (at high cost). Eventually parts started failing and factory stopped stocking parts for older machines. This is part of why Digital consoles got traction: the analog frying-fader technology just didn't work over the long haul, digital promised less to break.

[amptramp]

These were big news a while ago and Maxim probably has the widest selection of linear and audio taper:

http://www.maximintegrated.com/en/products/analog/data-converters/digital-potentiometers.html?gclid=CKqlm_yn9cICFY1cMgod1mAAPw

there is still stock of these:

http://canada.newark.com/analog-devices/ad5245brjz5-r2/ic-digital-pot-5kohm-256-sgl-sot23/dp/19M8811?CMP=KNC-GUSA-GEN-KWL-ANALOG_DEVICES&gclid=CNmcoaWo9cICFaZDMgodskEA5A

for $1.50 you can get this:

https://www.sparkfun.com/products/10613

[R.G.]

But the sweeter trick is to manually set the knobs, press "Store 13", and the CPU *reads* the pot positions to memory, "Recall 13" brings them back. Neither the servo nor the stepper can do that simply. A dual-gang motor pot "can", but the values are limited, and the values we may like for guitar-amp work are generally higher than what a deluxe hi-fi favors or what may be best for the CPU read-back.

Actually, that *is* what the stepper motor and dual pot does.

One pot is the actual pot, the second produces a position voltage. The uC reads the second pot voltage for "position". You can actually tell it "store 13", upon which command the uC reads the position voltage and stores it. Upon being told "recall 13", the uC reads the current position voltage and steps in the correct direction until the position voltage is closest to the stored value. I have built a uC position hunter with a pot for feedback, and they're easy enough to both implement and program.

The pot values are not practically limited, as using one pot for a voltage divider generally produces a voltage the uC can read OK, and even if that is a problem, an opamp follower after the pot wiper for high-value-pots lowers the impedance.

About the only issues with the stepper pot are (1) mechanics are a PITA (2) you have to do some not-too-taxing programming and (3) dual pots are difficult, as they require a three-gang.

Even this last has become moot. I ran onto a product from Bourns that amounts to a pot wafer with an attached wiper and a double-D hole in the wiper. It's intended to slip over a suitably shaped shaft and be turned by the shaft. You could gang any number of these on a single shaft, and the values don't even have to match. This makes the stepper-pot stupid-easy - well, OK, except for running the stepper.   

[PRR]

> I ran onto a product from Bourns that amounts to a pot wafer with an attached wiper and a double-D hole in the wiper.

Would like to see that part.

[R.G.]

I'll see if I can find it again. It came up in another search and I made a note of it for the next time I needed a shaft position indicator.  This was in the last year, so it's probably not out of production yet.

Edit:
OK, that wasn't hard. Look here:  http://www.bourns.com/ProductLine.aspx?name=contacting_single_turn

They're intended for shaft position sensing. I found them in a search for some electromechanical stuff, and noted that they're a solution to getting a semi-infinite number of non-identical section pots on a single shaft.

[PRR]

> Look here: http://www.bourns.com/ProductLine.aspx?name=contacting_single_turn

Thanks!!

Looks like a slick part for DIY pot-stacking.

Minor detail: hole is barely 1/8" diameter, can't simply shave a 1/4" rod. (However you could string them on a shaved bamboo skewer, then drill a pot-butt 1/8" and Sho-Goo it in.)

Major point: six parts in 3 values (100K, 10K, 2K5) are in distribution for small sales:
Mouser.com   "Bourns 3382"

Nominal price is $2-$3 each in onesies, but Newark shows the 10K 4mm part at $1.07 any quantity 1-250.
Newark.com   "BOURNS  3382"

[R.G.]

I vaguely remember more than one size available. You'd need to hunt.

And yes, a shaft could easily be wood. Actually, aluminum machines about as easily as some hardwoods. A 1/8" aluminum rod could be sanded flat on two sides with sandpaper glued to a board and get usable results if you were careful. Supergluing the rod to a holder block and then removing the glue with acetone should work.

I'm pretty sure that there are 1/4" to 1/8" shaft couplers available, but these days I'd probably just chuck up some 1/4" Al rod in the Prazi and center drill a 1/8" hole then epoxy in the 1/8" rod.

Either that or just cast a filled epoxy "adapter" with the 1/8" rod in the middle of a 1/4" OD bit of tubing.

[MetalGuy]

Just google "relay volume control" and you'll get plenty of projects. After motorized pots which are really a hustle the relay volume control is the next true analog way to build a digitally controlled pot. Depending on your needs you can build 4, 5, 6 or even 10 bit potentiometer. Of course you'll end up with dozens of relays that will take some space but there's always some price to pay keep it 100% analog.

[PRR]

There's two sizes of holes, 1/8" and a little smaller.

If there is another series which would take 1/4" rod, Bourns hasn't made it easy to find.

I did find an interesting video: a tour of a Mexican pot factory (specifically Bourns' fancy guitar-pot line).

http://www.bourns.com/ProAudio.aspx
Click "Vintage 82 Plant Tour"

It's rather talky and long-winded, with wild jumps in audio level (maybe they need a better pot?). But there are glimpses of the inker, the line into the kiln, the pin-swage, the automatic bushing-lathe, the Bourns-Blue molding machines, on to final assembly (by hand; and I swear some of the part-trays are cat-food/water dishes). Stop at 14:15--- they get into a bad guitar-jam and out-takes.

Bourns PSM is a console motorized slider with almost any value on the audio side, plus a servo track and touch-sense (used to kill the servo when the user wants hand-control). Rated motor speed about 0.3 Seconds end to end ("Flying Faders"!). DigiKey has 100+ in stock at ~~$20 but only the 10K B2 (modified Linear) job. I know Alpha or Panasonic has/had a competitive product.

[PRR]

me> Bourns PSM ... DigiKey has ...stock but..

A deeper search finds other PSM products listed but not stocked, including 200K and 1meg A4 (20% Audio), $13 each and minimum order 140 ($1,800!).