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Is there such thing as potentiometers that have a low resistance when centered and increasing resistances if you turn either left or right?

Edit: When i say centered, i mean that the pot is at zero resistance normally, when rotated clockwise, it will increase resistance. When rotated counter clockwise from the zero resistance position, it will also increase in resistance. Kind of like two pots put together in opposite directions.

Edit2: Well, specifically, I'm looking to modify a motor controller to give a nice smooth forward and reverse with a roll of the handle (think motorcycle throttle, except if you twist the other way, it reverses smoothly). I know the motor controller accepts a switch as forward/reverse and a pot as velocity control. Anyone ever encountered something like this?

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Can you elaborate on what you mean by centered? A lot of them will have a circular resistive strip that the wiper moves between two terminals as you rotate the control (which are attached to fixed points around the circle)... so it's all a matter of perspective really. –  vicatcu Jun 6 '11 at 21:27
What input does the motor controller use? –  endolith Jun 7 '11 at 18:33
Run a single pot into the A/D of a micro and sample. Set the bias point at VCC/2. If you sample higher than that go forward, if you sample lower than that, reverse. Much easier than a mechanical solution. –  Joel B Oct 3 '11 at 19:11
Thanks for the suggestions, though in the end, it became a mechanical hack with a basic 4 bar linkage system...only because the motor controller was OEM locked out and we couldn't reprogram it to do what we wanted it to do. Still, it worked much better than I expected. –  Faken Oct 4 '11 at 2:38
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4 Answers

up vote 3 down vote accepted

I'll suggest something wacky, in case you don't want to or can't modify your pots. How about connecting two standard potentiometers together with a mechanism, such that twisting a knob moves one pot, and twisting the knob in the other direction moves the other pot?

One possible way to do this is to have two linear pots with their grounded connections next to each other and electrically connected. Imagine a piece that goes between the two pots and it connected to both pot inputs via an elastomer (i.e. just wrap a rubber band around the pieces). When you move to the right, one linear pot will move freely, while the other is stuck against the hard stop. But because its connection to the input you're moving is elastic, the connection just stretches and doesn't risk breaking the pot. When you slide back in the other direction, the pot that was originally able to move will bottom out against its hard stop, while the one that was previously bottomed out will now move freely.

It's a little corny but I thought I'd throw it out there anyway. I can post a sketch if you like. :)

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Don't feel corny, what you describe is pretty much what I have modeled up right now. Being a mech engineer, its pretty much all I can come up with...but being an engineer, I also love elegant solutions. When you mix mechanical and electrical systems like that, it always just ends up looking like a hack job, but it works (and its inside a small metal box so no one has to even know about it). –  Faken Jun 7 '11 at 20:02
Here's a direct question about the subject: The motor controller takes 3 wires from the pot as input. Ground, +5, and wiper. I'm assuming it treats the pot as a voltage divider and analog reads the wiper voltage to determine speed. If I hooked up the two pots such that both pots share everything in common to the controller, what implications dose that have to the speed control, assuming what I posted earlier about how I am assuming the controller works...would this even work? –  Faken Jun 7 '11 at 20:08
Being a mechanical engineer as well, I didn't even think about the electrical end of your question, only the motion of getting some varied analog signal out of each pot, but not thinking about the electrical signal that needs to go into your controller. You technically could make this work by feeding your direction switch into an analog mux's address bus, and this would allow you to switch between the two pots. I've done something like that to hack together various feedback resistors for a school project, and it seemed to work okay for that. :) –  Dave Jun 8 '11 at 2:19
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There IS a potentiometer that does something very similar to what you want.
It is called a balance potentiometer.

Basically, it's used for exactly what it sounds like - Handling Left/Right balance in an audio system. It's effectively two potentiometers back to back, with unusual tapers.
When the pot is centered, both halves are close to zero ohms. When you turn it one way, the resistance on one half increases, when you turn it the other, the other half's resistance increases.

enter image description here

Bourns makes at least one here, but it is carbon composition, so not the highest quality..

I would imagine there are other manufacturers, but it is not a common item.

You are probably far better off using a plain old linear taper pot, and a MCU to handle the forward/reverse/dead-zone translation. Plus, that way you get adjustable acceleration curves.

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radio shack used to sell a balance pot, including a detent in the middle. –  markrages Jun 7 '11 at 17:09
Whenever we deal with audio pots, aren't they always "audio linear" which means that they are log pots? Or are these actually linear pots? –  Faken Jun 7 '11 at 19:58
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If you were to remove the stop on a potentiometer, so it rotates 360 degrees, then electrically tie the ends together, you would achieve something like you described. I've never seen such a pot for sale commercially.

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Im just getting into this. Would you care to clarify that a little more markrages? –  user6011 Oct 2 '11 at 15:45
@user6011: when you tie the ends together you have a two-contact (instead of 3) pot with resistance going from 0 to a maximum and then to 0 again. But you can't tell if you're turning left or right. –  Federico Russo Aug 16 at 12:44
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The J3R rocker pot has a spring return to center. I am using it in a project as a Wide-Tele remote control for a zoom lens. In addition to the wiper and two end-points, it also has a center tap which can be used to calibrate the null position.

I am feeding both the wiper and center tap into two channels of an ADC, and handling everything in firmware. But if you just look at the resistance between the two terminals, it works as you describe (0 ohms at center, increasing resistance either direction). You would need to compare the wiper with either one end or the other to determine direction.

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Interesting device. When I worked for Philips Audio we had something similar, but rotary (also +/- 15°). We called it a jog dial. May help others locate similar devices. I've also had Minolta lenses which used such a jog dial for motorized zooming and focusing. Very handy, but consumed batteries fast. –  stevenvh Oct 3 '11 at 11:49
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