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Thread: MIDI Faderbox fader choice

  1. #1

    MIDI Faderbox fader choice

    Hello,

    I am considering building a fader box for MIDI control around a Teensy 3.2.

    However I don’t know which fader brand/model to choose, starting with the value of the resistance, especially if I am to use the Teensy’s pullups.

    Requirements:
    - 100mm length;
    - linear curve;
    - low profile (the faderbox will be standing in front of a lying touchscreen);
    - clean output (to use the ADCs with as much resolution as possible);
    - edit: low consumption (I need the board to power 11 faders).

    Thank you in advance for helping.

  2. #2
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    The (internal) pull up resistors are only relevant for digital inputs.

    A standard value for this application would be a 10kΩ lineair slide potentiometer. ALPS and Bourns are widely used. The exact type would be dependent on your specific application (build) and budget.
    The 'Many_Button_Knobs' example in the Teensyduino install (under 'Examples > Teensy > USB_MIDI') contains what you need, especially the usage of the ResponsiveAnalogRead library. It is advisable to place a 10nF capacitor from the wiper to ground against RF and other interference.

  3. #3
    Quote Originally Posted by Gerrit View Post
    The (internal) pull up resistors are only relevant for digital inputs.

    A standard value for this application would be a 10kΩ lineair slide potentiometer. ALPS and Bourns are widely used. The exact type would be dependent on your specific application (build) and budget.
    The 'Many_Button_Knobs' example in the Teensyduino install (under 'Examples > Teensy > USB_MIDI') contains what you need, especially the usage of the ResponsiveAnalogRead library. It is advisable to place a 10nF capacitor from the wiper to ground against RF and other interference.
    Thanks a lot for this information. However I don’t understand why the pullups are not relevant for analog signals. You need a voltage divider to get the measure, don’t you?

  4. #4
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    Quote Originally Posted by amundsen View Post
    Thanks a lot for this information. However I don’t understand why the pullups are not relevant for analog signals. You need a voltage divider to get the measure, don’t you?
    The slide pot IS the divider: at the middle there'd be 5K above and below the wiper.

    Having the pull-up on would add a third resistor and make it less than linear. that might be desirable in some corner cases, but it's better to start simple. If you want a non-linear response, you can buy a non-linear potentiometer or fix it in software.

  5. #5
    Quote Originally Posted by drjohn View Post
    The slide pot IS the divider: at the middle there'd be 5K above and below the wiper.

    Having the pull-up on would add a third resistor and make it less than linear. that might be desirable in some corner cases, but it's better to start simple. If you want a non-linear response, you can buy a non-linear potentiometer or fix it in software.
    But what happens when the slider resistance is set to minimum? Isn't there a risk of short-circuit without a pullup? By the way my idea is not to have a non-linear response, but just not to break the board.

  6. #6
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    Quote Originally Posted by amundsen View Post
    But what happens when the slider resistance is set to minimum? Isn't there a risk of short-circuit without a pullup? By the way my idea is not to have a non-linear response, but just not to break the board.
    No, there's no problem. It is true that the analog input is shorted to ground when the slider is at the bottom end but that's not a problem because it is an input and there's no current flow from an analog input to ground.

  7. #7
    I completely forgot that a fader is already a voltage divider itself except when the wiper is at either end.

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