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Thread: Drawbars MIDI Controller planning questions

  1. #26
    Senior Member oddson's Avatar
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    AREF is an input for the voltage from an extenal reference voltage for the ADCs.

    Didn't notice the comment and didn't spot it when I looked it over.

  2. #27
    Quote Originally Posted by oddson View Post

    Nice.. looks good so far. Will be interesting to see your hardware.

    Just curious... will you have detents on your drawbars/faders?
    I don't know if that's still a thing but I believe some organs did as I recall playing with a home-style Hammond organ from '50s or '60s and my recollection is detent settings to call up the 'recipes' from a sounds book.

    BTW - You got me wondering about the merits of SPI vs I2C; found an interesting reference:
    http://www.byteparadigm.com/applicat...spi-protocols/
    I see only now that my reply to you (posted from my cellphone) was lost.

    I too I'm curious to see how the hardware will come out... from my experience in previous projects the HW part is often overlooked both in terms of feasibility and in terms of costs...
    Now that all of you have helped me achieve some confidence on the electronic part I'm starting the layout part .. I'll keep you posted

    About the drawbars potentiometer: the only ones I found are sold here and I don't think they are dented.
    In a previous project I used a real drawbars assembly from a real Hammond organ, with some resistors added, but for this project I wouldn't go this way for 2 main reasons: the first is that in Europe finding dismantled Hammond parts is quite difficult and so they are way too expensive for such a small project, the second is that the drabars I adapded are not providing reliable end constant values when I use them as Analog input.
    It could be that, at least for the first iteration, I'll use standard slide potentiometers like they used on the Ocean Beach DB-1 controller

  3. #28
    Quote Originally Posted by oddson View Post
    AREF is an input for the voltage from an extenal reference voltage for the ADCs.

    Didn't notice the comment and didn't spot it when I looked it over.
    OH I see! Thank you very much!
    I'll soon post an updated schematic so if someone else will look here he'll not be confused

    EDIT: schematic updated in post #20
    I also attach it here
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by garubi; 10-05-2016 at 04:08 PM. Reason: added correct schematic

  4. #29
    Senior Member oddson's Avatar
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    I've never used it but that's my understanding... if you have a sensor that outputs over a specific voltage range or you are dividing voltage from an external source you need to provide the ADCs the reference voltage.

  5. #30
    Another little question.
    In my schematics I've put a 4.7k Ohm resistor between both the 2 i2c lines and 3.3v as suggested in the Wire reference page, but this post seems to suggest that 4.7k is more adeguate for 5V while for 3.3v 2.2K Ohm is better.

    I'm really not sure I understood that post, maybe it's a whole different situation, so any hint is well appreciated

  6. #31
    Quote Originally Posted by oddson View Post
    AREF is an input for the voltage from an extenal reference voltage for the ADCs.
    That's exactly what I thought but since I am a noob and no-one said anything I assumed I was wrong in some way. Feels good to be useful at times.

  7. #32
    Senior Member+ MichaelMeissner's Avatar
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    Cool

    Quote Originally Posted by garubi View Post
    Another little question.
    In my schematics I've put a 4.7k Ohm resistor between both the 2 i2c lines and 3.3v as suggested in the Wire reference page, but this post seems to suggest that 4.7k is more adeguate for 5V while for 3.3v 2.2K Ohm is better.

    I'm really not sure I understood that post, maybe it's a whole different situation, so any hint is well appreciated
    4.7K is the standard resistor used for 5v systems. It will run on 3.3v volt systems, but you might not be able to set the i2c speed to faster speeds. If you never set the i2c faster than 100Khz, then it probably does not matter for small i2c setups with only a few devices and short wires.

    From what I've read of other posts over the years (I'm a software guy, not an EE) the default value to use for 3.3v is 2.2K. If Paul or Nox771 say otherwise, follow their advice, and ignore mine, since they are the i2c experts. I started using 2.2K over 4.7K for i2c, when Paul released the Prop Shields, and those had 2.2K resistors in them.

    In general, you only want one set of resistors on the i2c bus. If you are hooking up devices with their own set of resistors, you might not want to add additional resistors (for example, as I mentioned, the prop shields have 2.2k resistors built-in).

    In complcated i2c setups, this document is one of the ones to consult to calculate the best resistors to use: http://www.ti.com.cn/cn/lit/an/slva689/slva689.pdf.

    This post shows the effect of varying i2c resistance: http://www.dsscircuits.com/articles/...l-up-resistors.
    Last edited by MichaelMeissner; 09-27-2016 at 03:54 PM.

  8. #33
    Quote Originally Posted by MichaelMeissner View Post
    4.7K is the standard resistor used for 5v systems. It will run on 3.3v volt systems, but you might not be able to set the i2c speed to faster speeds. If you never set the i2c faster than 100Khz, then it probably does not matter for small i2c setups with only a few devices and short wires.

    From what I've read of other posts over the years (I'm a software guy, not an EE) the default value to use for 3.3v is 2.2K. If Paul or Nox771 say otherwise, follow their advice, and ignore mine, since they are the i2c experts. I started using 2.2K over 4.7K for i2c, when Paul released the Prop Shields, and those had 2.2K resistors in them.

    In general, you only want one set of resistors on the i2c bus. If you are hooking up devices with their own set of resistors, you might not want to add additional resistors (for example, as I mentioned, the prop shields have 2.2k resistors built-in).

    In complcated i2c setups, this document is one of the ones to consult to calculate the best resistors to use: http://www.ti.com.cn/cn/lit/an/slva689/slva689.pdf.

    This post shows the effect of varying i2c resistance: http://www.dsscircuits.com/articles/...l-up-resistors.
    Thank you Michael!

  9. #34
    Senior Member oddson's Avatar
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    Quote Originally Posted by garubi View Post
    Another little question.
    In my schematics I've put a 4.7k Ohm resistor between both the 2 i2c lines and 3.3v as suggested in the Wire reference page, but this post seems to suggest that 4.7k is more adeguate for 5V while for 3.3v 2.2K Ohm is better.

    I'm really not sure I understood that post, maybe it's a whole different situation, so any hint is well appreciated
    Here's an application note from TI on I2C pullup selection:
    www.ti.com/lit/an/slva689/slva689.pdf

    There's a bunch of math in there if you want or you could try the 4.7KOhm and drop the value from there if it turns out not to be enough pull to bring the voltage back up fast enough for the protocol to work in your specific case.
    Quote Originally Posted by Texas Instruments
    Once the minimum and maximum value of the pullup resistor has been selected, the decision for the value
    of resistor can be made based on trade-off between the speed and power budget. A smaller resistor will
    give a higher speed because of smaller RC delay, and a larger resistor will give lower power consumption.
    Presumably Paul has found 4.7K still works at the lower voltage in most cases or the wire page would offer different values.

    (edit - The article also covers pullups for multiple devices.)

  10. #35
    A little update: I changed the schematic I posted before, now with the correct potentiometer-to-3.3V connection.
    I'm now looking online to find all the components, and in the meantime I'm planning with SketchUp the enclosure.

  11. #36

    Cool Project completed!

    Just a quick note (I hope that I'm not breaking any forum's rule of conduct) to let all of you know that the drawbars MIDI controller (proudly named MIDI Drawbars Commander ) is almost complete!

    Click image for larger version. 

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    I really want to thanks all the forum members, not only who directly helped me (well, Paul as the creator deserve a special mention ): even if I posted just a bunch of questions I learned a lot reading all of you.

    When I started this project I didn't know anything about Teensy, Arduino, C++, etc... and now, not only this device is accompaning me in many gigs around the country, but I also completed several little projects involving the same technologies.

    If you want to take a look, I'm documenting the whole project at hackaday.io: https://hackaday.io/project/162255-m...bars-commander

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