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Thread: Hardware questions

  1. #1

    Hardware questions

    Hi,

    I'm doing a pcb for a Teensy 3.5 eurorack module project and I have some questions about hardware.

    I plan to have many potentiometers (18), is it better to use a multiplexer or using 18 analog pins is fine ?

    Are these schematics right ?

    Click image for larger version. 

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    Thanks for your responses

  2. #2
    Member houtson's Avatar
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    hi @xtreemechill

    I'm no expert but a few points just from my experience with pots and audio effects:
    - are you using the audioboard as part of your design, if so remember that will take out a few analogue pins
    - same for the DACs if you are planning on using them
    - is this a one off, if it is and you are ok soldering onto the bottom of the Teensy there are a few extra analogue pins
    - what accuracy are you aiming for - I've never needed and external reference board but you may need more accuracy that I've needed
    - I've used 10 pots directly onto the teensy (no ref voltage, no multiplexer) with no problems at the same time as running effects and lights etc. 10 wasn't a limit it was just all I needed

    cheers, Paul

  3. #3
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    I'd start by seeing if you have enough pins to use rotary encoders instead of Pots. They produce much more stable and repeatable results. You might be able to do that by using pins on the T3.5's back side.

    This breakout makes accessing the back side pins much easier: https://www.tindie.com/products/logl...on-a-standard/

    Since you're designing a PWB anyway, maybe you can accommodate it?

  4. #4
    Senior Member+ Frank B's Avatar
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    Yes, but use quality encoders. The cheaper ones tend to bounce a lot, so the repeatability takes time and they have to be turned left-right again and again to get the right value...
    Or be prepared to add a debouncing technique if necessary...
    All in all, some pots might be easier to use. That depends on the accuracy you need.

    You can't use the ADC for Audio if you use pots.

  5. #5
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    The State Table Approach nicely handles contact bounce as part of its algorithm.

    Here's one implementation that I wrote: https://github.com/gfvalvo/NewEncoder

  6. #6
    Senior Member+ Frank B's Avatar
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    Or this: https://forum.pjrc.com/threads/60483...rary-debounced (uses a table, too) which is 100% compatible to the std PJRC Encoder library.
    But it seems that it is not compatible to every kind of encoder. I don't own these not-working-encoders so I cant do anything to fix it.

  7. #7
    Member houtson's Avatar
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    The problem I've had with encoders for audio effects/applications is the lack of resolution with a single turn - with 24 pulses per revolution you need to turn the knob 10 times to get 8 even bit resolution (256 steps).

    My experience is they're good for menu selection and things like that but prefer a pot if trying to adjust frequencies or other audio parameters where you want to cover a large range in a single turn with some accuracy.

    If you could get a small and cheap 512 or 1024 step encoder that would be ideal but haven't found any of them.

  8. #8
    Senior Member+ Frank B's Avatar
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    Quote Originally Posted by houtson View Post
    The problem I've had with encoders for audio effects/applications is the lack of resolution with a single turn - with 24 pulses per revolution you need to turn the knob 10 times to get 8 even bit resolution (256 steps).

    My experience is they're good for menu selection and things like that but prefer a pot if trying to adjust frequencies or other audio parameters where you want to cover a large range in a single turn with some accuracy.

    If you could get a small and cheap 512 or 1024 step encoder that would be ideal but haven't found any of them.
    Audio settings are rarely linear. Especially the volume.
    Normally logarithmic curves are often used.

  9. #9
    Member houtson's Avatar
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    Quote Originally Posted by Frank B View Post
    Audio settings are rarely linear. Especially the volume.
    Normally logarithmic curves are often used.
    I agree but I still want more that 24 steps per turn

  10. #10
    Senior Member+ Frank B's Avatar
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    If you find an old mechanical mouse, you'll get two optical encoders..

  11. #11
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    Quote Originally Posted by houtson View Post
    prefer a pot if trying to adjust frequencies or other audio parameters where you want to cover a large range in a single turn with some accuracy.
    I find that the apparent setting hysteresis, drift, and lack of repeatability inherent in using pots outweighs other considerations. But, YMMV.

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