Connecting Multi-Turn 50K pot

#1

Hi,
I was testing a 50K 10-turn pot I had laying around here today. I bought it something like 15 years back, and could not find any datasheet of this specific model..
(I would like to use it to make some kind of Ribbon Martenot alike instrument with a string..)

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I connected the outer contacts to VDDA and GRN, middle to GPIO PB0.
It seems that the readings on PB0 are 10x smaller than they should be.
Measuring the pot with a multimeter shows correct behaviour. (except for a spike at the end of the last turn, but that is what most of these MT pots do apparently..)

Anybody experience with reading out multi-turn pots?
I guess it is no coincidence that the values are /10...

Any help greatly appreciated!

#2

There should, of course, be no difference between a multi turn pot and a 'normal' pot. Have you tried a normal pot?

Are you certain you are using the correct connections on the MT pot? I'm wondering if, maybe, you are using the wrong pin as wiper?

#3

Hi Teleplayer,

I have tried a normal 50K pot and everything works fine with that one..
I am quite sure I use the correct connections since the measurements with the Multimeter are fine..
hmm. Not sure what is happening here..

#4

just to state the obvious, a multiturn pot will actually output 1/10th of it's range in one turn if it is a 10 turn pot, not 10 times the full range on each turn. but i guess that you already know that...

#5

Hi Lokki,

indeed, what I see is around 1/10th of the range for the full 10 turns...
I must be making a stupid mistake here..

#6

That's what I'm trying to determine.
Assuming you've tried a normal 50K pot, on the same Axoloti connections, with expected results, the only mistake I can think of is using the incorrect connections on the pot.

So, you wrote "the measurements with the Multimeter are fine", I'm guessing you didn't do that test correctly. A resistance test between the two outer pins on a pot should be constant, about 50K, regardless of wiper location. Once you've found the pins that satisfy that, the remaining pin is the wiper.

If this is obvious, forgive any offense - it's not intended. But a mistake is being made, somewhere.

#7

if all suggestions fail, can you try and read the voltage at the wiper pin with a multimeter, with vcc and gnd connnected to axoloti. does that work? if so, maybe try a different GPIO. do you have a pullup/down connected (internal maybe)

#8

I think the pot must be faulty..

Reading between one of the outer pins and the wiper I get perfect readings ranging from 0 to 50K until the end where there is a sudden totally closed.
Reading between the two outer pins results in totally open for the full range until the end where there is a sudden totally open... EDITED..

@lokki: I have nothing connected, just gpio analog in and disp/p.. I tried different gpio too.

EDIT: Reading between the two outer pins results in totally open for the full range until the end where there is a sudden totally CLOSED...

#9

Hi Lokki, what do you mean by internal?
tnx for taking the time!

#10

Maybe the pot is damaged or defective.

Just to be clear - the wiper is the pin nearest the back of the pot, NOT the center pin.

#11

Hi Lokki,

you are right. The image printed on the pot mislead me. I think I must have fried it during my first attempt to connect it..
Things learned!
Thanks a lot from a learning noob :wink: