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I am trying to find a component, that, similar to a potentiometer, delivers an analog output, but which can be turned indefinitely in a single direction.
I have tried searching for something like this, but I never quite found anything that does what I want, while being relatively cheap.

Please note, that I am not searching for a rotary encoder, since they either are clicky, do not have the resolution I require, do not support absolute positioning and are usually too expensive if they do support any of these.

I imagine something like a normal potentiometer with multiple wiper contacts might work in this case.

This is the diagram of a normal pot.

This is what a pot with 2 wipers could look like.

It would provide a smooth precise output even when turned in a single direction for multiple turns.

Does a cheap part exist that behaves like this and if so what is it called?

Edit: thanks /u/quetzalcoatl for providing the output potential diagram.

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    \$\begingroup\$ IME rotary encoders are not expensive, and can be 'un-clicked' with some effort. \$\endgroup\$ May 13 '18 at 15:52
  • \$\begingroup\$ While this can be easily done for a small amount of products, it would be rather cumbersome to do it for a larger amount. Also, most commercially available rotary encoders only have some ~ 20 steps per turn, while a pot is technically stepless, and a 10-bit ADC can easily read 1024 steps per turn. Most commercially available µC have multiple ADC inside that could be used for this purpose. Also please note that I do not want to use this at very high speeds. All turning will be done by a Human hand. \$\endgroup\$
    – Busti
    May 13 '18 at 16:15
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    \$\begingroup\$ can you add the angle/resistance relationship you are after? to me it is not extremely clear. \$\endgroup\$ May 13 '18 at 17:17
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    \$\begingroup\$ What resistance do you expect to observe after turning the pot an infinite number of times to the left? \$\endgroup\$ May 13 '18 at 19:26
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    \$\begingroup\$ Somewhat off-topic but in my experience you can always get rotary encoders with or without detent (what you call clicky), and the 24-angle encoders can be used to get 96 unique angles. \$\endgroup\$
    – pipe
    May 13 '18 at 19:27

12 Answers 12

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The device Spehro Pefhany's answer builds out of two pots is actually available as a single unit, for example the ALPS RDC803101A.

If you turn it, you get two sawtooth signals that are 180° out of phase, so when one output is in its “dead zone”, you can use the other one instead to determine the position.

This model also has no detents, so no clickyness at all.

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    \$\begingroup\$ Nice find... +1 \$\endgroup\$ May 14 '18 at 5:53
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    \$\begingroup\$ +10 This should be the selected answer. Nothing to beat a consumer part that is made to do the exact function desired. Cost and size are spot on and ALPS is a reputable supplier. \$\endgroup\$
    – KalleMP
    May 14 '18 at 7:29
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    \$\begingroup\$ This solves my question. The other solution was the best one so far, but this is exactly what I need. Great find. \$\endgroup\$
    – Busti
    May 14 '18 at 15:48
  • \$\begingroup\$ It was very much a specialty item, but in the 60s you could get a 360 degree pot with a sine output. It had 5 terminals, call them sequentially 1,2,3, 4.and 5. You'd tie 1 to a + reference, 3 to an equal but opposite - reference, and 2 and 4 to ground. 5 would produce a sine wave with shaft rotation. Accuracy would be on the order of 0.1%. Real expensive, though. \$\endgroup\$ Aug 1 '20 at 1:02
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The best continuous rotation sensor I've used is the AMS series.

Something like this might suit your purpose. enter image description here

Of coursed they can't support infinite output values, but associated with an MCU you can set it to mid scale each time you turn your unit on, or remember last settings.

There are also plenty of relatively cheap optical encoders that would allow the same strategy of infinite rotation. For example, this from Bourns

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  • \$\begingroup\$ An array of hall sensors is also a great solution to this problem. It might even be cheaper than the double sided pot solution, since it does not require an assembly on the bottom of the pcb \$\endgroup\$
    – Busti
    May 13 '18 at 21:35
  • \$\begingroup\$ Unfortunately these require the knob or stem to be magnetic, which would increase manufacturing complexity. Usually the housing of a pot also serves as an anchor for the shaft, these would require the products casing to do that job. \$\endgroup\$
    – Busti
    Jul 31 '20 at 23:53
  • \$\begingroup\$ @Busti magnetic 'po't replacements are available, as are incremental and absolute encoders. In the magnetic versions the pot shaft is NOT magnetic, the magnet is attached to the internal end of the shaft. tinyurl.com/yy66zp5j tinyurl.com/y593teum \$\endgroup\$ Aug 1 '20 at 16:11
  • \$\begingroup\$ @Jack_Creasy I would not have guessed that parts like that exist. Thanks a lot for the comment! \$\endgroup\$
    – Busti
    Aug 1 '20 at 20:48
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I have not seen something like that, does not mean that it does not exist. It does not particularly lend itself to construction with normal pot designs, including modular ganged pots, because the leads usually come out of one side.

There are pots without end stops, however they have a dead angle. Expensive long-life ones are called 'servo pots'.

You could make such a device by mounting two of these pots on opposite sides of the board (phased differently, obviously).

enter image description here

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    \$\begingroup\$ That pot is a great recommendation. But I wonder how much this increases the cost of manufacturing a board with it, since one of them has to be mounted on the opposite side of the pcb, preferably with the shaft already inside, so that they do not misalign. \$\endgroup\$
    – Busti
    May 13 '18 at 15:56
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    \$\begingroup\$ Cheapest in NRE would be to use single sided assembly and attach the bottom one by hand with the shaft inserted. For higher volumes - it's not uncommon to machine fixtures for reflow soldering, particularly when double sided assembly is used. For example \$\endgroup\$ May 13 '18 at 17:22
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What you may find useful is a multi turn potentiometer. Bourns manufactures them in different ranges of resistance. They come in various sizes, small and volume knob size too.

They help in precise control of resistance. There are others too, with a dial inside and outside too, from Bourns.

I could get a big multi turn potentiometer for my project for about 2 Dollars, and the small ones are 0.2 Dollars or so.

Here's what they look like... These have 10 turns to vary a given potentiometer value. So for a 10k potentiometer, you get 1k variance for each turn.

multi turn potentiometer

multi turn potentiometer

enter image description here

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    \$\begingroup\$ multi-turn is not continuous-turn. \$\endgroup\$ May 14 '18 at 6:08
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How "clicky" was the ball movement on an old-school ball mouse? That uses a rotary encoder. It's only a simple 2-bit one though, which is enough for speed and direction of motion, but does not give an absolute position.

Do you actually need an absolute position though? This really comes down to your application. If cost is an issue, you can also drastically reduce that in the same way as mouse manufacturers did, by making the encoder wheel part of the mechanism and just using a simple break-beam slotted disk for the encoder.

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For completeness, sin / cosine potentiometers are also available. They are fairly rare and expensive nowadays.

A quick Google search found these:

  • State electronics SPSC50S 2" Dia. Servo Mount Potentiometer
  • Atheris FSCB22A 23 mm diameter servo mount sin/cos potentiometer

among others.

In general, potentiometers have fallen out of favour is position measurement devices. This is because they tend to have inferior lifetime, resolution, linearity and cost compared to encoders, despite their significant advantage of being 'absolute' rather than 'incremental'.

This writer spent many an hour in his relative youth replacing noisy and/or non-linear position readout pots in medical systems. The replacement pot then had to be adjusted, calibrated for gain and offset, and verified for linearity.

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I believe this part did exist once, and they just used a stereo pot (has two separate cards), with one wiper rotated, and no stops.

Unfortunately I have no proof, so it could be a figment of my imagination.

If you want to try your idea, it's easy to get the top section open, and reverse the card.

enter image description here

You can't just reverse this wiper as it is keyed with 120deg tags enter image description here

This pot has a separate pcb with the track on. This means that the wiper rotates one way but not the other. You want a pot where the track is printed on the card.

Unfortunately on this pot, the stop is in the bottom section, but you can get that open, push the stop in the picture out, and reassemble with the card and case half rotated 180deg.

enter image description here

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The Alpha RV112FF may be what you're looking for? In short, it's an endless dual potentiometer where both tracks produce a triangle shape as they're rotated, and they're offset by 90 degrees, so by reading both positions, you can deduce the absolute position.

The correct google term seems to be "sin cos encoder", which finds plenty of hits, although many of them point to industrial units which have several cycles of sin/cos over their rotation, kind of combining a normal encoder and a potentiometer...

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  • \$\begingroup\$ That is a great search term. That pot is also very nice, but the cycle count is a bit low. The other one linked in the accepted post has higher cycle count, allthough it is not much. I intend to use this for a human input device that is supposed to be fiddled with constantly, so having a higher cycle count would be desirable. \$\endgroup\$
    – Busti
    Mar 30 '20 at 13:41
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the RV112FF from Taiwan Alpha (http://www.taiwanalpha.com/en/products/4) is such potentiometer with 360 degree endless rotation. I found this while searching for a LONG time for drivers for such potentiometers. I could not find any algorithms to decode them, so I have made one. Hopefully this can be useful for you others looking for same. https://hackaday.io/project/171841-driver-for-360-degree-endless-potentiometer)

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  • \$\begingroup\$ Do you have any experience with the Alpha part? I am a bit concerned since their expected lifespan seems to be a bit on the low side. Do you happen to know which parts were used in the KORE parts you tore down? \$\endgroup\$
    – Busti
    May 22 '20 at 20:36
  • \$\begingroup\$ I do not personally have any experience with Taiwan Alpha. There was no marking on the ones in the KORE controller identifying the brand, but I read in a few other online discussions pointing to the Taiwan alpha ones (metal shaft type). There has been some complaints about the pots "jumping around" on the KORE, and some claims contact spray improved the issue. As with any pot it will wear down over time, but for my use I only need values of 0-127 (MIDI). And I am sampling them with 10 bit DAC with hysteresis/filtering so it should be able to compensate even for fairly worn down/noisy pots. \$\endgroup\$
    – bbrandal
    May 26 '20 at 2:24
  • \$\begingroup\$ Thank you very much for your insight. I ordered some TaiwanAlpha pots from mouser to try them out. What I really care about is absolute positioning, that's why rotary encoders won't do for my project. \$\endgroup\$
    – Busti
    May 28 '20 at 16:29
  • \$\begingroup\$ One of my KORE pots stopped working, so I bought one of these Taiwan Alpha ones. The one I bought from Mouser was RV112FF-40B1-15F-0B20K. Notice that this is a 15mm shaft ("-15F") while the KORE ones are 20mm shafts. But it does not matter, the knobs sits well on the 15mm shaft as well. And as far as I can tell the Alpha pot is otherwise identical to the original ones. Just notice that the new one feels much tighter when rotating, as expected from a brand new pot.SO if you want to have same rotational resitance on all pots you probably should replace them all. \$\endgroup\$
    – bbrandal
    Jul 2 '20 at 2:13
  • \$\begingroup\$ @Busti, how did you find the TaiwanAlpha pots? (I assume you mean the RV112FFs.) I'm not expecting my users to twiddle constantly, more like every five minutes they'll twiddle a half a turn. \$\endgroup\$ Oct 5 at 12:42
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You do not state the application. If your goal is a volume control on an amplifier so that your gangsta rap can be turned up to infinite volume, I can't help you. But if your goal is to detect the position of a rotating shaft, you might consider synchro transformers. They give an x and y output, where x is the cosine and y is the sine of the rotation angle. They can be turned forever (or until they wear out), and there are no wipers to go bad. Of course, it is best if the excitation frequency is not lower than the shaft rotation frequency ;-).

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  • \$\begingroup\$ I personally need it to control color on an HSV color wheel. So for my application just the angle is of importance. But I also want it to be cheap and rather small. The phase offset pot is probably the best solution for me, but your answer makes me wonder how a shafts revolutions can be reliably counted at very high speeds. \$\endgroup\$
    – Busti
    May 18 '18 at 21:07
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    \$\begingroup\$ My experience with synchro resolvers goes back to the days of analog flight control computers. Angle of attack sensors, aircraft control surfaces, and gyroscopes did not make revolutions at high speeds. But if you need to count the revolutions at high speed, the logic for any of the above solutions is similar. Note my remark that the excitation should be a higher frequency than the shaft rotation; otherwise you get something called ALIASING. It is like when the stagecoach wheels or airplane propellers are turning backwards in an old movie. \$\endgroup\$ May 21 '18 at 17:38
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The 1-turn 10K pot linked below (UXCELL, on Amazon) includes a small indentation on its metal case that acts as a stop. By drilling out the indentation with a 1/8" bit, you can convert it into a continuously rotatable pot without having to disassemble the pot. To avoid getting bits of metal inside the pot, drill the stop material until it is fairly thin and then pull the remaining stop material out of the indentation with needle-nose pliers. Each turn of the modified pot produces a nice 0-1023 reading at an Arduino analog input.

https://i.stack.imgur.com/Fq6RS.jpg

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    \$\begingroup\$ Isn't there going to be a 30° deadspot where the wiper will be disconnected from the track? You'll only get 0 to 1023 over a span of 330°. You'll also need either a pull-up or pull-down to maintain the ADC input at a fixed value rather than floating. Welcome to EE.SE. \$\endgroup\$
    – Transistor
    Oct 6 '20 at 18:28
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There are 360 degree linear pots that you can get without rotation stops, like http://www.bourns.com/docs/Product-Datasheets/6630.pdf

With these, there is always an interruption, so continuous rotation at constant velocity will produce a sawtooth.

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