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Thread: Requesting help - T3.1 MIDI controller schematic

  1. #1
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    Talking Requesting help - T3.1 MIDI controller schematic

    Code:
    echo 'Hello World!';
    First microcontroller project, and first post in these forums...

    I set out on an adventure to build my own MIDI controller and now I find myself here- with a box of parts, a schematic, and a dream. I've read plenty of pages on other peoples' projects, and after many revisions, today I finally drew out a draft of how I think everything will fit together. That's where you come in....

    Having no experience with this type of project before, I want to triple-check everything and make sure I'm not going to wind up with a pile of fried components. I was hoping that someone might be kind enough to proofread the attached diagram and tell me what mistakes I've made (and maybe even suggest how to fix them...).

    The original [of this compressed image] is a 5.3mb PDF that was too large to upload here, but I can send it by an alternate means if this is illegible.

    Click image for larger version. 

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    The diagram shows more potentiometers than I actually plan on connecting. I just kinda got carried away when I was connecting everything and sketching it up. I also haven't decided on a specific NPN transistor; I'm not really sure what would be best for this specific application- or even if that is the best option.

    Any thoughts, comments, advice, or critical feedback welcome!



    Thanks in advance

  2. #2
    Senior Member PaulStoffregen's Avatar
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    I'd recommend testing just one of those RGB LEDs. The resistor values seem to be computed for fairly high current. The recommended maximum current for a Teensy 3.1 pin is 9 mA. The "absolute" maximum is 20 mA.

    I don't quite understand what's meant by "latching" switches.

    The 74HC4051 seems to have 8 switches connected with resistors in series. The switches connect to +3.3V. Maybe those resistors where means to be pulldowns to ground, rather than in series?

    Pins 7 and 8 on the 74HC4051 appear to be unconnected. Obviously those need to connect to ground for the chip to work. It's also good practice to place a small ceramic capacitor between 3.3V and ground, located close to each chip. Typically 0.1 uF is used, but the actual value is less important that using a ceramic type with good high frequency performance and locating it close to the chip, so the wires do not add much inductance that limits the frequency response. If the chip needs current at high frequency (for example, when the digital signals change), short connections to a local capacitor helps a lot.

    You probably don't need a fan.

  3. #3
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    Hey! Thank you so much for your [extremely] rapid reply. I'm reeealllly glad you proofed my work..... sheesh. I'm totally new to electronics... can you tell?

    I'd recommend testing just one of those RGB LEDs. The resistor values seem to be computed for fairly high current. The recommended maximum current for a Teensy 3.1 pin is 9 mA. The "absolute" maximum is 20 mA.
    Yes, I calculated the resistor values based on 20-25mA. I tried to measure the current [literally the first time I've used that feature of my multimeter....] and on the 200mA setting it read either '.3' or '.6'. I figured that either meant '.3/.6mA' or '30/60% of 200mA'. Apparently I picked the wrong one.

    If I recalculate for 1mA:

    R: 1150 (1200) ohm
    G: 300 (330) ohm
    B: 1400 (1500) ohm

    5mA:

    R: 230 (270) ohm
    G: 60 (68) ohm
    B: 280 (330) ohm

    I tested both and will probably go with the latter, if that won't burn them out too fast.

    I don't quite understand what's meant by "latching" switches.
    Poor use of terminology on my part... I just meant toggle/rocker switches, as opposed to momentary push buttons.

    The 74HC4051 seems to have 8 switches connected with resistors in series. The switches connect to +3.3V. Maybe those resistors where means to be pulldowns to ground, rather than in series?
    Yes, that makes more sense. I misunderstood the page I was reading that described using that chip. On that same note, I should probably add pulldown resistors to all of the digital inputs so they aren't 'floating', right?

    Pins 7 and 8 on the 74HC4051 appear to be unconnected. Obviously those need to connect to ground for the chip to work.
    Ahhh- yeah, when I traced the schematic in pen so it would scan better, I forgot to trace over the pencil line that had pin 8 attached to ground. I did not have pin 7 connected, though. This morning I've done a little bit of reading on 'Vee' but I don't think I understand the why of it; I'll keep working on that!

    It's also good practice to place a small ceramic capacitor between 3.3V and ground, located close to each chip. Typically 0.1 uF is used, but the actual value is less important that using a ceramic type with good high frequency performance and locating it close to the chip, so the wires do not add much inductance that limits the frequency response. If the chip needs current at high frequency (for example, when the digital signals change), short connections to a local capacitor helps a lot.
    Would this be an appropriate ceramic capacitor? http://www.digikey.com/product-searc...ds=399-4266-ND


    You probably don't need a fan.
    I'm sure you're right on this; I've already prepared the case to accommodate a tiny one so I'll probably keep it in anyways, but add an on/off switch so it isn't running by default.

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by whiterabbit View Post
    This morning I've done a little bit of reading on 'Vee' but I don't think I understand the why of it;
    TL;DR = just connect Vee to ground for this project.

    The 74HC4051 chip is actually capable of conducting negative voltages, if properly powered by two power supplies: a positive and a negative power supply. The maximum voltage it can conduct is limited by the positive power, and the minimum voltage is limited by the negative power supply.

    Obviously you don't need this feature, since your signals are only from 0 to 3.3V, and the Teensy is only capable of reading positive voltages. So just connect Vee to ground.

    The ability to conduct both positive and negative signal is useful for some projects, like connecting different audio signals to an amplifier. Of course you could design such a project without the negative voltage by using a capacitor to couple the AC signal to a fixed DC offset voltage, so the entire signal is always between 0 to the power supply. But being able to directly work with signals that range from positive to negative eliminates the need for such design tricks which come with other trade-offs. The Vee pin and special circuits inside the 74HC4051 allow you to work directly with those types of signals.

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    TL;DR = just connect Vee to ground for this project.
    Haha, ok. I can definitely do that. Thank you for the explanation, that makes more sense.

    In reviewing my plans more, I realized that I also screwed up with the transistors; I have common cathode RGB LEDs, and from what I've been reading I should use PNP transistors instead of NPNs as I had planned.

    Headed back to the drawing board for now, which will give me time to get those capacitors and more resistors of the proper values to use for all the pulldowns.

    Will check back in when I've made more progress....

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