Auto reset knob

[st.mu]

Does anyone know how to make a knob automatically spring back to a certain position, similar to a pitch wheel or joystick? I thought there are special rotary potentiometers for this but I couldn't find any parts generally available. Is this usually done as a modification on an existing potentiometer?

[PRR]

Welcome.

Can you point to any commercial product with a knob like this? I have not seen one. Aside from, as you say, joysticks.

[FUZZZZzzzz]

I once opened a mod wheel i found. I wanted to build it inside a pedal. I found a normal pot inside but there was also a small spring inside the modwheel that turned it back to its starting place.

[iainpunk]

the mod wheel on a keyboard is often a digital encoder, not a pot.

if i were to need such a function, i'd use a slider pot with pair of springs, which would be totally slack when the pot is at either end of travel and they'll even out in the middle.

cheers

[Kevin Mitchell]

the mod wheel on a keyboard is often a digital encoder, not a pot.

Idk about that statement. I've yet to see an encoder in a pitch or mod wheel assembly. Even a midi controller would use an analog to digital converter and map out the 1024(for 10 bit) values to 127 for midi.

Here's a couple photos from my favorite wheel assembly from the Pro2021 kit - from the build thread on another site. Should give a good idea of how the center-return typically functions (could grab better photos from mine if you'd like)

[kaycee]

Another way to do it is with two knobs for the same function and a momentary switch that selects the second setting while held down and returns to original setting on release.

[PRR]

the mod wheel on a keyboard is often a digital encoder, not a pot. ....

May depend on your age. In the ARP 2500 it was surely a pot. But the fanciest "digital" in that beast was a 4017 decimal counter.

Now I see what st.mu is thinking of and I've never seen it as a catalog part, except as a joystick.

The two-pot plan from kaycee is intriguing.

[iainpunk]

the mod wheel on a keyboard is often a digital encoder, not a pot.

Idk about that statement. I've yet to see an encoder in a pitch or mod wheel assembly. Even a midi controller would use an analog to digital converter and map out the 1024(for 10 bit) values to 127 for midi.

i have taken apart 3 keyboards, not midi, but those that have everything build in, and 2 of em had modwheels and both were encoders with springs. i might have a sample bias, as by sample is small and the keyboards were cheap. i probably should have typed '' the keyboards i took apart both had encoders''

cheers

[ElectricDruid]

i have taken apart 3 keyboards, not midi, but those that have everything build in, and 2 of em had modwheels and both were encoders with springs. i might have a sample bias, as by sample is small and the keyboards were cheap. i probably should have typed '' the keyboards i took apart both had encoders''

I'm amazed. I've taken apart dozens of keyboards over the years, and like Kevin, I've never seen one with an encoder. No particular reason why you couldn't use an encoder, but a pot is simpler, generally. One ADC input for a pot beats two digital inputs with debouncing and decoding needed for an encoder, in terms of IO pins and complexity. Except apparently in some gear where there must be no ADC on the chip, so they found another way.

The typical way to do the "return to centre" is to add springs, so yes it's a modification to an existing pot. Some use the kind of springs Kevin showed from the Pro2021, some use the kind with little loops on the end. Generally for those, you have two, with one getting stretched when you push the wheel up, and the other stretched when you pull it down. When you let go, the stretched spring pulls it back to the centre. Either method is not exactly super-sophisticated, but it's the kind of thing that needs either machined parts or perhaps decent work with a 3D printer.

[blackieNYC]

I was going to suggest something like kaycee's solution, but the stomp switch would disconnect the pot and give you a fixed pot resistance of your choosing.  Depending on the feature being controlled, you might not get a pop. 

[Mark Hammer]

In pawn shops and 2nd hand stores everywhere, there are older "car racing" games that come with two spring-loaded foot treadles, one for gas and one for brakes.  if they were for a PC of some kind, chances are pretty good they used 50k pots.  The gist was that the spring would return each pot to some default value, read as "no gas" or "no brakes applied".

Another approach is that using a force-sensing resistor, where the resistance value is reduced when you press on it, and it goes back to maximum value when you lift your finger.  Possible that the default no-pressure value may take longer to re-establish than the application calls for.  David Rainger uses them for his effects.

[anotherjim]

I don't know of any ready-made sprung solution for a normal panel mount rotary pot. Probably Doepfer or similar will sell a synth sprung mod pot assembly but that won't be good fit. I can imagine a possible DIY solution involving a coil spring as pictured above with the tails bent outward pointing along the pot shaft axis in opposite directions and poking into a hole in the enclosure top and the bottom of the knob. The spring coil would have to be wound in the right direction to bias the knob the way you want it to sping back to.

[MrStab]

Some digital pot ICs have a simple up-down interface, which could be used without a microcontrollers.
Some digital pot ICs have a pin which snaps the wiper back to the middle position.

These Venn circles do not overlap.

[PRR]

...Doepfer or similar will sell a synth sprung mod pot assembly....

https://doepfer.de/zubeh_e.htm
"Keyboard accessories
Modulation Wheel kit:"
Price (Euro) 15,00
https://doepfer.de/pdf/Modulationsrad_Montage.pdf
"Pay attention that not the full rotating angle of the potentiometer is covered but only about 40%."

[Kevin Mitchell]

I agree with the suggestions to use a stomp switch with your centered value as it's super simple. With a mechanical center return you're going to want a spring connected to the actuator itself which is not possible as a discrete solution.

Perhaps you could make the mechanical profile low enough to put under the knob but I wouldn't call this a safe method. I have a thought of doing this with a hollowed out ring underneath the skirt of the knob with one peg on the knob and another on the enclosure, but installing the mechanism (being a rubber band or two) would be damn near impossible.

You could find a long shaft pot, mount the bushing below the surface of the enclosure via a custom bracket and place the return mechanism underneath the surface. This would also include a custom collar that mounts to the lower part of the shaft with a setscrew (or knurled collar if you're one of those knurled weirdos) to mount the spring or rubber band.

Just use the switch lol

[Kevin Mitchell]

i have taken apart 3 keyboards, not midi, but those that have everything build in, and 2 of em had modwheels and both were encoders with springs. i might have a sample bias, as by sample is small and the keyboards were cheap. i probably should have typed '' the keyboards i took apart both had encoders''

Didn't mean to throw ya under!
Just something I haven't seen myself after repairing a few synths & organs and reviewing many a dozen different service schematics. However I did own a couple controllers that used softpots where the pitch "center return" was programmed.

[kaycee]

The two-pot plan from kaycee is intriguing.

Here is how I do it using a DPDT momentary footswitch for a tremolo circuit. You can go from fast to slow and vice versa, circuit returns to original pot 1 rate when switch released. You need more than two poles if you need to change more than 2 connections. Instead of pots, you could have fixed resistors (or selections on switches) or capacitors as well.

[ElectricDruid]

"Pay attention that not the full rotating angle of the potentiometer is covered but only about 40%."

This is a fair point if you're using sprung-to-the-centre mod wheels. Modern synths have calibration routines to deal with this sort of thing, but if that's not the way you're heading, it's much more awkward to deal with.

I wrote a little PIC routine to deal with this on an expression pedal (same situation) to make sure it gave me a 0V to +5V output for the travel from full-down to full-up.