Semiconductors...

Discussion in 'General Electronics Chat' started by TheBellows, Jan 28, 2009.

  1. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    Hi, i'm a new member here and a noob when it comes to electronics.
    I have done circuit bending for a while and i'm now into learning and experimenting with audio circuits.
    So far i'm building simple oscillators, preamps and other audio circuits where i can find the parts i need for building...anyway, enough about me and over to my questions...

    The other day i found this cool thing on the net somewhere, don't know if you're familiar with it, but they call it a "synthstick".
    It's a variable resistor that changes the resistance depending on where you touch it.
    The idea is to lay a strip of mylar and a strip of vhs tape on top of each other without connecting, so that when you press down various places you get connections between the two materials, resulting in variable resistance because vhs tape is a semiconductor....
    But ther is my problem. My vhs tape doesn't seem to conduct at all, not even with the test pins 1mm from each other, nada.
    How can this be? I've seen people on the net do this...is there something different with the vhs tape i'm using?

    As the vhs tape didnt work i tried to find a replacement for it and the only thing i could think of was carbon from a pencil drawn all over on a strip of paper.
    I replaced the mylar strip too, with aluminumfoil glued on a strip of sticky tape.

    These materials worked to some extent, but it was very difficult to get a linear resistance with the carbon. As a prototype i'm quite happy with my half a meter 50K pot "slider", but It's rather useless when it comes to controlling oscillators for musical purposes. ;P

    Any ideas to what i can use as the semiconductor strip?


    -Ragnar :)
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How long do you want to be your "touchstick" ?
    A layer of carbon could do the trick, like in a slider potmeter.
    The touching layer must be conductive.

    For some nice musical schematics i have a link for you.
    http://www.musicfromouterspace.com/analogsynth.php?page=ANALOG

    There you can find all kinds of oscillators mixers vca's and so on.

    Greetings,
    Bertus
     
  3. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    0
    That was excactly the way i made my prototype, the "half a meter 50k pot" if you read at the bottom of my post ;P


    I know i can make it with carbon, but i really want a more stable semiconducting material, like a foil of some sort, something not so fragile (a pencil stroke changes the resistance drastically just by rubbing it).

    I hoped i could get this VHS-tape thing to work, but for now it doesn't seem like that is gonna happen...

    Thanks for the link, seems very interesting :)

    -Ragnar
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Do you know the "Theremin" ?
    It is a very old musical instrument.
    It uses two antenna's.
    One for the volume and one for the hight of the tone.
    The volume and pitch is changed by moving the hands.

    Perhaps you can use the principe for your application.

    Here is a link to some schematics.
    http://www.thereminworld.com/schematics.asp

    Greetings,
    Bertus
     
  5. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    0
    Yes i know the theremin, and it's one of the things i'm aiming for, but i need several big copper winded coils and i'm not shure if i'm able to wind them myself at this point, all these Henrys and stuff... :p
    I made some oscillators controlled by a LDR, it's fun too, but now i really wanna make a "keyless keyboard" ;)

    -Ragnar
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Perhaps you can take some experimetal PCB.
    Solder resistors between the lines and you will have a strip of resistors.
    Place a conductive wire over it.
    When you press the wire on the PCB, the resistance will depend on the place the wire touches the PCB.

    Here is an example how you could do it.

    [​IMG]

    Each line I have drawn on the PCB represents a resistor.

    For more information on resistors look at this page :
    http://www.educypedia.be/electronics/resistors.htm

    For information on the coils for the Theremin, take a look at this page :
    http://www.educypedia.be/electronics/coilsrelay.htm

    These links come from the EDUCYPEDIA :
    http://www.educypedia.be/electronics/electronicaopening.htm

    Greetings,
    Bertus
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A quick question - did you have the dull sided of the recording tape strips in contact? The shiny side is bare plastic.

    And did you use something like Silverprint paint to make the external connection?
     
  8. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    As to the pencil carbon, what grade pencil did you use? Did you try an artist's pencil (7B, maybe?) The softer the pencil, the more carbon in it.

    Also, I remember a DIY resistor made by cutting the wood from one side of a pencil, attaching a wire to the point, and sliding a wire up and down the exposed carbon shaft.

    Happy Tinkering!
    --Rich
     
  9. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
    56
    0
    There are no dull side on my vhs tape, and i've checked both sides in any way i can think of with an ohm meter. No conduction.

    Not shure what silverpaint is and what you mean by external connection, but that gave me an idea...maybe there is some type of metallic paint i could use instead of the vhs tape or carbon?
    I will check it out if i find some metallic paint laying around...coal mixed with glue pherhaps?



    Thanks for the suggestions guys.

    I used different types of pencils from 4H - 6B to a thick carbon stick. The problem with carbon is like i said earlier, it's difficult to make an even layer, it becomes unlinear. The carbon is not stuck to the paper either, some carbon come off here and there and it's just not a stable material to make precision instrument.

    The strip board idea is good, but it kinda takes away the purpose of making it without keys in the first place. The point is that i want to be able to slide my finger on it with no gaps between the notes, just a touch based potensiometer.


    :)

    -Ragnar
     
  10. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    0
    Now i tried the inside of an old discette and some other vhs tapes, metallic spray paint and ...no conduction there either.

    Anyone who knows if there exists some type of wire with a semiconducting property of something like 200K-1M ohm per meter?
    Then i could use several wires like on a guitar, with different fixed or variable resistors connected to each wire (then i could tune the strings like a guitar too...a guitar that never get out of tune :p ) . I would need to tighten the wires so they have to be strong though...


    Is magnet band a semiconductor?
     
    Last edited: Jan 29, 2009
  11. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    0
    I'm not actually talking about semiconductors am i?
    I think i might be mixing some terms here, english is not my native language and nor is my electronic language as you may have guessed already.
    I'm actually a barbaric viking from Norway, and here we have no time to go to school becuse we have to go over seas to plunder villages and rape women while drinking mead and eating fly agaric mushrooms. :p

    -Ragnar
     
  12. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    0
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The writer of the article is right.
    Some tapes are made of some ferryoxide others from ferry with chrome and others from a chromelike material.
    Some are conductive, some not.
    The old brown ones (also cassette tape) ypu could try to measure.

    Greetings,
    Bertus
     
  14. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    Seems like none of the tapes i have is the one i need, aren't there any other solutions?
     
  15. bertus

    Administrator

    Apr 5, 2008
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  16. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
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    Thanks i will look into it :)

    I would like to make a real synthstick with a large resistance too though, i like it for testing purposes and making wacky sounds with my silly 555 circuits :p
     
    Last edited: Jan 31, 2009
  17. TheBellows

    Thread Starter Active Member

    Jan 18, 2009
    56
    0
    Starting to think that this resistive tape thingy does not excist, i have checked around 50 VHS tapes (none of them had a dull brown side) and lots of casettes from my days of commodore 64 and other music casettes (some which had a brown dull side). None of them conducted anything at all. :(
     
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