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A/D keyboard

Discussion in 'Electronic Design' started by Phil, Jan 10, 2004.

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  1. Phil

    Phil Guest

    I want to implement a keyboard with 7 switches
    and need to use an A to D. Seems staightforward
    except I need to detect if more than 1 switch is
    pressed at the same time. So I will need a ladder
    with 0.5,1,2,4,8,16,32 for the resistor values, I plan
    to wire 7 resistors in series across each swiches and
    feed the A to D from RX/R1 junction.
    Has anyone done this before and any idea what values
    to use for RX and RY with R1 = 500, R2 = 1K ...R7
    = 32K ?

    VCC--RX--R1--R2 --R3--R4--R5--R6--R7--RY --VSS

    Phil
     
  2. Jim Thompson

    Jim Thompson Guest

    Sounds like you are trying to make a D-to-A. Look up R-2R Ladder.

    ...Jim Thompson
     
  3. mike

    mike Guest

    This MIGHT be a bad idea. Do the math. Look at the
    real resolution/accuracy of your a/d (not just the number of bits),
    resistor tolerances, noise
    immunity requirements, definition of "at the same time",
    what happens if the keys are misinterpreted? Also look at how much
    overhead you'll incur polling the A/D.
    My gut tells me you want a different implementation.
    A one-wire-type interface with a OTP microcontroller
    might be cheaper than the parts you need to make the a/d scheme work.

    Now, if you had two a/d inputs and could arrange the switches so
    the simultaneousness is only two at a time and always spread across two
    inputs, it becomes
    a slam dunk for tolerances and simultaneousness.
    mike


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  4. James Meyer

    James Meyer Guest

    Good call Jim. I recommend that Phil gets 25 or so 100K resistors of at
    least 1% accuracy. The R-2R can be made with all one value of resistor and if
    they all come from one batch then they will be very likely to track one another
    in value. It's only the ratio of R to 2R that really matters and not the actual
    R value. Any value from 1K to 1Meg would work. I picked 100K to make things
    easier on the reference supply and still allow reasonable bias currents for the
    following op-amp.

    Jim
     
  5. Jim Thompson

    Jim Thompson Guest

    In the past year I've gone bananas over potentiometric DACs... at the
    IC level who cares that it takes tons of resistors... I've even made a
    two-stage 14-bit one that's been reduced to practice.

    How are your ribs ?:)

    ...Jim Thompson
     
  6. GPG

    GPG Guest

    7 switchesWhy? Be more specific about application.
     
  7. Alan

    Alan Guest

    Have a look at:
    http://www.st.com/stonline/books/pdf/docs/2484.pdf


    HTH
    Alan


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  8. James Meyer

    James Meyer Guest

    Ribs are almost no problem now. Best estimate I've gotten so far is
    that ribs take two months to heal 100%. That makes mine about 50% well.

    I found out last week that my arm was broken too. They told me at the
    ER right after it happened that I had one broken rib and a sprained arm. After
    looking again at the X-rays later, they said *two* ribs and a chipped bone
    somewhere in my arm.

    All things considered, I can think of several better ways to spend a
    holiday.

    Jim "on the mend" Meyer
     
  9. Phil

    Phil Guest

    Alan
    I need to read more than one switch and
    and the ST solution is only for a single keypress.
    But thanks for your suggestion.

    Phil

    ----- Original Message -----
    From: "Alan" <>
    Newsgroups: sci.electronics.design
    Sent: Sunday, January 11, 2004 3:39 AM
    Subject: Re: A/D keyboard
     
  10. Phil

    Phil Guest

    Thanks Jim, spot on.

    Phil
     
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