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Thread: Problem with pot center value

  1. #1
    Junior Member
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    Nov 2019
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    Problem with pot center value

    Hi,

    I'm working for a few days on a MultiFX box with a Teensy 4 + Audio shield.
    For the FX controls I based my work on the schematics available on Teensy site :

    Click image for larger version. 

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    My config is quite similar, i got 3 74HC4051, 1 linked to a digital pin (9) for arcade buttons and 2 linked to analog pins (16,17) for pots.
    Arcade buttons have a led in it that I connected to digital pins (4,5,14,15) to turn on and off.

    Here is the panel layout of the prototype box :

    Click image for larger version. 

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    Everything is working quite well except that my pots are jumping the center values. All pots are jumping from ~500 to ~650 when I try to center them.
    I'm using 100k linear pots.

    I measured the voltage coming out of the pots with a multimeter and I read values from 0 to 3.3v with apparently no jump on center as seen once I read values with the teensy.
    I'm using ResponsiveAnalogRead to smooth the values but I also tried to check raw values and I see this jump too.

    I think I saw a similar problem on the forum, advicing to put capacitors on the circuit but that was not really clear to me.

    Could someone give me some advices about this issue ?

    Thanks for your help.

  2. #2
    Senior Member oddson's Avatar
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    100 kOhm is very high especially for mux'd pots.

    It could also be insufficient settling time between setting and reading the mux.

    A cap on a mux'd pot is bad as it slows settling time.

    https://forum.pjrc.com/threads/32373...ll=1#post92634
    Last edited by oddson; 11-30-2019 at 03:30 PM.

  3. #3
    I had a similar problem and found out that, with high impedance inputs, the 74HC4051 do not work well. This is related to the design of the chip, which leads to midrange errors, if the chip runs on 3.3V unipolar (Vcc=3.3V and Vee=GND). It works as expected with Vcc=+5V and Vee=-5V, but gives a risk of overshooting or undershooting input specs of Teensy. Also, there's a difference between HC and HCT, with the HC being a lower-voltage type.

    The 74HC4067 perfroms as expected in unipolar mode, even at 3.3V and high impedance and is very well usable with high impedance. The Teensy just needs a longer sampling time set in the sketch. So an impedance of even 3MOhm works, at the cost of ADC conversion speed. Also, the setting time of the switch needs to be awaited in the sketch, before sampling is initiated.

  4. #4
    Junior Member
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    Nov 2019
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    I just made a few tests with 10K pots and there is no jump at the center with these ones

    Problem solved, many thanks for your help.

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