Replace volume control of USB speaker with a potentiometer

[WT87]

Hi everybody,

In a 'robot' I am building there is a Raspberry Pi integrated with some buttons.
Those buttons light up LEDs, power a servo and play sound through a USB speaker.
I have bought USB speakers with a volume control on it knowing I could replace it with a potentiometer which is attached to the belly of the robot.
The problem is that I do not know how to replace the default flat potentiometer with the new 'normal' potentiometer. There are more connections than three on the 'normal' potentiometer and I am not familiar with technical implementations.

I have attached some photos to make it more clear.
This is the potentiometer I have: https://www.adafruit.com/product/562
These are the usb speakers, Audiocore AC870 (Dutch website): https://www.bol.com/nl/p/compacte-stere ... oductTitle

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IMG_20190122_093445.jpeg (39.38 KiB) Viewed 1235 times

Thank you in advance for the help!
Wouter

[Burngate]

... There are more connections than three on the 'normal' potentiometer and I am not familiar with technical implementations.

I have attached some photos to make it more clear.
This is the potentiometer I have: https://www.adafruit.com/product/562

By "the 'normal' potentiometer" do you mean the one built-in to the speakers? If so, I can't quite see where this is in your photos.

However, I'm guessing that it's actually two potentiometers mechanically joined together, so you'd need a dual-ganged pot such as https://www.adafruit.com/product/3392

Be aware, though, that the original pots may not be 10k, and they may not be linear.

[LTolledo]

does the "flat" potentiometer have 5 or 6 pins?

if you can get the part number of the "flat" potentiometer, then a little bit more web search (now most people refer to this as "goggling") will lead you to a data sheet....

... and as mentioned by Burngate... the potentiometer may not be "linear"... could be logarithmic... only the data sheet can clear that up.

[WT87]

@Burngate,
The 'normal' potentiometer is the https://www.adafruit.com/product/562
This was not included in my photos.
I guess there is only potentiometer on there, the black wheel.
It indeed could be not 10k or linear. This is something I just read about after your and LTolledo's comment.

@LTolledo
The "flat" potentiometer has 5 pins. See photo.
Unfortunately I can not see a part number on it or find it through the company's website or part maker's website...

I guess it's not linear since it's working with audio and increasing audio volume. (or is that too fast of a conclusion?)
Searching on the various numbers on the board with potentiometer does not bring me anywhere useful unfortunately.
Are there any specifc forums or manuals I could dive in to in order to get the part number and thus datasheet?

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IMG_20190122_125625.jpg (94.94 KiB) Viewed 1197 times

[hippy]

The "flat" potentiometer has 5 pins. See photo.

I would guess it's an integrated dual-gang pot -

Code: Select all

)------------.
)---.    .---|----(
    |    |   |
   | |<--'  | |<--(
   |_|      |_|
    |        |
    `--------^----(

A resistance meter should be able to determine if that's the case and which connector goes to where.

[Burngate]

Sorry, got confused. It was too early in the morning for rational thought.

Hippy's response is the most valuable.
Given that the underside of the board has LIN, RIN & GND labelled, you're half way to finding out which pin is which in his diagram

[LTolledo]

Looking at the Top side and underside pictures of the pcb, this is what I can tell

pot.jpg (83.01 KiB) Viewed 1161 times

[drgeoff]

That is a stereo pot - the L(eft)IN and R(ight)IN markings on the board in are a clue! Usual two connections for the track ends and one for the moving contact per channel. The groundy end of both channels can be commoned. Hence 5 pins. If you look closely you may be able to see two wipers and two concentric tracks.

Use an ohmmeter to find the resistance. Most likely to be log rather than linear. Ohmmeter can check that too. Put the knob to mid position. If resistance from wiper to one end is significantly different from half the end to end value then it is log.

[WT87]

I tested the resistance between the out and in of Lin and Rin afterwards.
The multimeter is set on 20K Ω.

One end: 0
Halfway: 15
Other end: 1.75

The difference does not look staggering different from each other but I guess it still an log. Am I correct?

That means I have the buy this potentiometer: https://www.adafruit.com/product/3394
Although I don't see at the moment there must a datasheet of this.

[LTolledo]

Testing the pot's resistance is best done if the pot is removed (desoldered) from the PCB. Doing so will also reveal the numerical value printed on the back side.

Once you've confirmed the value of the pot, then next is ordering the replacement pot.

You may also consider this form factor for your pot's replacement

minipot.jpg (55.27 KiB) Viewed 1063 times

[IanS]

I am guessing that you made a typo, and that 1.75 should have been 17.5. The halfway reading should be between the two end readings.
As already mentioned, measuring the values with the pot in place may not be completely accurate, as there may be alternate connectivity paths.
Measure the resistance between the ground connection and one of the in connections. This will give you the maximum resistance. Ground to either in connection should be the same, within the tolerance of the device. Then measure resistance from ground to one of the outs, when the dial is set to halfway. If the pot is linear, then this will be about half the first measurement. If the pot is log then it will be about 30%, or 70% if you have measured resistance between in and out rather than ground and out.
15/17.5 is about 85%, which is a good indication of it being log, the setting not being quite at halfway, the tolerances being crap and possibly other components still connected effecting the reading.

[WT87]

Hi,

I have got the potentiometer out and it is a B503.
I noticed before taking it out one of the legs on top was connected to ground. This can be seen on the photo on the green side of the PCB below Lin and Rin.
It's this one: https://www.ebay.com/itm/20PCS-B503-16x ... 1#viTabs_0
The datasheet I haven't found yet.

I took the following readings based upon your advice:

200k resistance on multimeter

Ground/in R
End 56.6
Middle 56.6
End 56.6

Ground/out R
End 0
Middle29
End 56

Ground/in L
End 52.7
Middle 52.6
End 52.6

Ground/out L
End 0
Middle 30
End 52.6

It feels the pot might be linear.

@LTolledo
Is there any difference between this potentiometer and the one I send earlier as an option or just the shape of it?

[drgeoff]

I'd wager B503 means B law (which is log) and 503 means 50 kohms.

[StuartF]

B is Linear
A is Log

[drgeoff]

B is Linear
A is Log

That does not seem to be a universal convention.

See http://www.farnell.com/datasheets/2711323.pdf pages 4 and 5.
See http://www.farnell.com/datasheets/14851.pdf page page 2

as evidence that at least 2 different manufacturers use A for linear and B for log.

[LTolledo]

I prefer linear, others prefer log.
Selecting linear vs logarithmic pots will be up to the user's preference on how he/she wants the volume controlled.

Whether you accidentally pick the other, just adjust the volume to the level to your liking....
....you have full control over it

[Burngate]

Our ears are more-or-less logarithmic, so log volume controls are better.

With linear controls, going from a mere whisper to too loud takes just a few degrees of rotation, with the rest of the turn doing not very much.
Log-law pots stretch the lower bit to more of the turn, enabling more precise control.

[WT87]

Since it's concerning audio control I'm going for the log potentiometer.
It makes most sense and it's good way for me to play around with it.
It's going to be for a 'robot' and not a grand audio system so that lowers the bar for me.

I'll keep you guys posted if interested and perhaps I'll have some followup questions.

Thanks for the help so far!
Greetings from across the tiny pond