newbie voltage /resistor network question

Discussion in 'General Electronics Chat' started by phidelity, Nov 2, 2008.

  1. phidelity

    Thread Starter New Member

    Nov 2, 2008
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    hello

    i am a musician with an elementary knowledge of electronics and i am working on building a gesture sensor network for my instrument that will convert dc voltages of the sensors feeding into an interface which will convert the voltages to midi messages.

    i will be using one of a number of commercially available interfaces that will manage that aspect of the problem. my problem is i want to build a series resistor network that will vary an input voltage (ranging from 0 to 5 v dc) dependent on which fret i close the circuit on. i will apply a voltage to the base of the strings and mount the resistors on the top edge of each fret and as i move up the neck of the instrument the voltage will change. from this the sensor interface will be able to map specific values to midi messages.

    i spoke to an electronics tech at a local shop and he told me that i by building a resistor network i would be varying the current not the potential but the difficulty is that all the interfaces use this industry sensor standard of 0 - 5v dc. so can anyone point out some good reading to clarify how i can manage this problem or point me in the write direction on how to solve this. or enlighten me on how to vary voltage rather than current.

    here are a few links to the paper on the controller i am using as a model and to some of the standard interfaces. just in case i have left out any relevant info or am a little vague.

    thanks

    phi

    http://www.mistic.ece.uvic.ca/publications/2004_dafx_Esitar.pdf

    http://www.electrotap.com/teabox/
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Can you post a diagram of what do you think to do?
    Also, what exactly is a fret on a musical instrument?
     
  3. phidelity

    Thread Starter New Member

    Nov 2, 2008
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    hi mik3

    if you check the link to the pdf from my previous post you can see a picture of what the author had done which hopefully clears things up.

    A fret is the metal bar that provides the spacing for moving up the scales on a guitar, bass and in my case sitar. on my instrument there are 20 which allows the player to move two octaves up from the lowest note to the highest.

    because they are metal and conductive the goal is to put a resistor on each fret as i depress the string to play a note the charged string makes contact with that specific fret and the current flows through the resistor(s) to the control interface. the idea of having the resistors in series is to have a different voltage (amperage - not sure ) for each fret. so if i play the lowest note the charge has to flow through all the resistors but if i play the highest note the charge only flows through the first resistor. so i want a different voltage for each fret to identify the note played.

    http://soundlab.cs.princeton.edu/research/controllers/esitar/

    here is another link with a few more photos.

    thanks
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Ok, connect the resistors as you think in series as many as needed for the frets plus another resistor in series with them on the side where the ground of the circuit will be. Your output voltage will be the voltage across this extra resistor. Also, connect your strings to the positive voltage of the power supply. If you press the first fret the voltage will be divided across two resistors only (high), if you press the second one the voltage will be divided across three resistors (lower than before) and so on. If you want the lowest voltage to be when you press the first fret just flip the whole resistor structure.
     
  5. steveb

    Senior Member

    Jul 3, 2008
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    If you are a musician, I'm sure you have heard of electrical accidents related to musical equipment. You should make sure that you fully understand the safety issues with regards to musical instruments. This can get tricky, and I'm even afraid to make recommendations. I'm sure you are thinking that the 5V level is probably safe, which it generally is, but the player is going to constantly be touching those strings and will be at risk if things are not wired properly.
     
    Last edited: Nov 2, 2008
  6. phidelity

    Thread Starter New Member

    Nov 2, 2008
    3
    0
    if you have a relevant link or a suggested resource i would love to read this to be certain i haven't made any errors in logic. BTW i like your signature - good quote!


    2) to mic3 - if i understand your post correctly you are saying the voltage will drop according to the structure of the resistor network (as would the current) and the extra resistor on the ground circuit is to make certain i don't have a short circuit when stroking an open string.

    thanks for you input.
     
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    Hi, "Phid"

    This is a technique that was done in the original Moog analog synthesizer...but with a keyboard instead. You're going to need PRECISION resistors....but it's definitely doable....if Moog could do it, so can you.

    You want to build a string of resistors...if you've got a two octave neck, you'll need 24 of them...in series. Each fret will tie to a junction of a resistor pair.

    The string, when pressed against the frets, will short out, more or less of the resistor string. The total resistance will be what determines a voltage going to a VCO (voltage controlled oscillator). The real art to this is coming up with resistor values in the right progression....it's going to be a LOGARITHMIC change of resistance per fret.

    It may not be as big a problem, however, if you use a string of comparators...one for each fret. That way you simply have an ON or OFF state for each comparator. Then you can do the fine tuning in software.


    Eric
     
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