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Assembling a digital voltage display

Discussion in 'Electronic Components' started by JoeSpareBedroom, May 4, 2007.

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  1. I need to assemble a digital voltage display for, of all things, a bass
    guitar which uses two 9v batteries to supply power for tone control
    circuitry. I need some tips. The problem to be solved is this: Unscrewing
    the battery hatch in the instrument is no big deal, but checking the
    batteries involves flexing the wires, and this has caused failures twice in
    the past. So, I'm thinking of mounting something like this on the surface of
    the instrument:

    http://www.radioshack.com/product/i...port=support&numProdsPerPage=20&tab=techSpecs


    Short link:
    http://tinyurl.com/2yt6mx


    I'd actually need a display which also showed a decimal point and tenths of
    volts, but the example should be enough to illustrate the basic idea here.
    In addition to the display, I'll probablly want to mount a button or switch
    to disconnect the batteries from the circuitry, and enable the display just
    long enough to check voltage. I need to measure as low as about 6.5 volts,
    at which point the circuitry ceases to operate, and any lower voltage is
    irrelevant.

    So:

    1) If components like the LED display could operate without a few other
    parts involved, that would be too easy, right? If someone can recommend a
    decent book which might contain info on what I'd need to add, in order to
    make this work, that would be great. (I'm old - I know how to operate a
    library card).

    2) I haven't checked all 28 billion google results yet, but if anyone can
    suggest a better source than Radio Shack for LED displays, that would be
    great.

    Of course, I could just mount a banana plug jack on the face of the
    instrument, and plug in a voltmeter when necessary, but that's one more
    contraption I don't have room for on top of the amp during performances.
     
  2. Tim

    Tim Guest

    Any device you put on to monitor your batteries is going cause more
    drain. Instead of LED type, you might want to consider LCD, or perhaps a
    low battery warning with no display, something that beeps when you need
    to replace the cells.

    My 2 cents worth anyways... (and that's CDN cents too...)

    - Tim -
     
  3. LCD would be fine. A bit less visible, but not a big problem. As far as the
    beep idea, it would never be heard during a performance.
     
  4. Tim

    Tim Guest

    Well my idea of a low battery warning, would be at the point where you
    would have less than a few hours left on cells, not when you had a few
    minutes, so if you had a beep before the set, you changed batteries. A
    beep after or during the set would not be a concern, as you would have
    enough power to finish the set.

    - Tim -
     
  5. ehsjr

    ehsjr Guest

    That's your best option. Get some Velcro with
    adhesive backing, and hang the voltmeter on the
    side of the amp.You can get a dirt cheap voltmeter
    on sale from Harbor Freight that will be fine for
    this function. It's on sale now for $4.99 - in the
    past it has gone for as low as $2.99. ITEM 90899-2VGA
    http://www.harborfreight.com/

    Ed
     
  6. Guest

    Why so complicated? Why not a good old d'Arsonval movement? Drains
    microamps.
     
  7. Needle type meter? I wondered about that, but I also wonder about
    durability. The bass doesn't get banged around much, but still....

    And on the other hand, I've met old Simpson meters that never seem to die.
     
  8. Gary Tait

    Gary Tait Guest

    If you want a small battery level meter, a simple bar meter will do (button
    activated) , or even a simple comparater lighting a "battery good" LED, or
    a "battery low" LED

    You could esaily attach a coaxial jack on each device you wish to test, and
    use an external tester that would be no bigger than a 9V battery.
     
  9. I was thinking this too. Never came up with the goods though so I let it
    go... I was thinking of MOSFETS. There might be a way to keep one turned
    off when the gate voltage is high enough, so that the total current draw is
    in the nanoamp range, but the LED begins to glow more as voltage falls.
    Obviously the limited remaining charge will be used even more rapidly, but
    usefully, and no significant drain would begin till there was a need to
    indicate. I'd hoped to work out something with one FET, an LED, and no more
    than two resistors, but it eluded me. Maybe it can't be done that simply,
    but it seems like the way to go, to get as close to that as possible.
     
  10. rpbc

    rpbc Guest

    by "JoeSpareBedroom" <[email protected][EMAIL PROTECTED] > May 5, 2007 at 03:16
    PM


    <[email protected][EMAIL PROTECTED]
    Needle type meter? I wondered about that, but I also wonder about
    durability. The bass doesn't get banged around much, but still....

    And on the other hand, I've met old Simpson meters that never seem to
    die.

    rpbc: Why not build a regulated 9 volt supply and feed it from the power
    supply of the bass amp... or just buy a little wall type transformer of
    the right voltage wired to the bass amp power switch, and add filter and
    regulation to that? Then you won't have to worry about the battery.
     
  11. Because I play 99% of the time with a Sennheiser wireless setup. I move
    around too much to have wires underfoot.
     
  12. rpbc

    rpbc Guest

    by "JoeSpareBedroom" <[email protected][EMAIL PROTECTED] > May 12, 2007 at
    09:36 PM


    "rpbc" <[email protected][EMAIL PROTECTED]
    .com> wrote in message
    Because I play 99% of the time with a Sennheiser wireless setup. I move
    around too much to have wires underfoot.

    rpbc: Oh, I see. Think I only got half the picture. Could you install a
    little female jack on the compartment wired to the battery and have a male
    plug on the leads of a voltmeter so you can just plug in an measure? You
    could measure with the unit on so the reading will be under load.
     
  13. That's the solution I'm leaning toward right now. The compartment is
    precisely big enough for the two 9v batteries. I'm taking the bass to a
    luthier later this week to discuss the safest direction in which to rout out
    enough space for a banana jack, without threatening the integrity of the
    wooden body.
     
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