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Resistive tape

Discussion in 'Electronic Basics' started by lerameur, Oct 25, 2006.

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  1. lerameur

    lerameur Guest

    Hello,

    Anybody can tell me where I can buy resistive tape. Any code numbers at
    Digikey. I do not mean electrical tape. I want to use it in an toy
    elevator, and will use it to calculate the precise height of an object,
    mainly a long voltage divider scheme

    thank you

    ken
     
  2. Tom Biasi

    Tom Biasi Guest

    You used the word "precise", your method is not going to give you precise
    results.
    There are a number of ways to get pretty accurate results, the best for the
    dollar is probably something like counting bars on a strip or powering the
    unit with a stepper motor and tracking the steps.
    Tom
     
  3. DJ Delorie

    DJ Delorie Guest

    Can you use a carbon lead from a mechanical pencil?
     
  4. Potentiometer with a string and a pulley? There are multi-turn
    potentiometers out there, by the way.

    Optics? Gray code, or a simple bar/stripe counter?

    Precise ... aah, that is the real question ... how accurate does this have
    to be?
     
  5. Yukio YANO

    Yukio YANO Guest

    Why not just rework a computer mouse! Everything you need is there, you
    could even use a spare mouse to troubleshoot the software/hardware. It
    would eliminate the need for an A/D converter.

    Yukio YANO
     
  6. If you want precise, code a strip with punched holes with say 4 holes across
    and use binary encoding. At each step or floor a set of four photo
    transistor read the binary number encoded on the strip. That way you just
    use a geared motor or a stepper and have it stop when it finds the number at
    the strip. JTT.
     
  7. lerameur

    lerameur Guest

    well I want to be precise. i will need it to stop at every 5mm, and on
    the way down ned it to stop at the same spot.
    Maybe use the same thing they use in wheel encoders ?

    ken
     
  8. It is a for a toy so I think he might need to use the newest cutting edge
    lasers.
     
  9. Chris

    Chris Guest

    Hi, Ken. Easiest way is to use a gearmotor with a cam to trigger a
    microswitch to stop the motor after every turn. You can use a relay to
    override the cam to start for the next turn. I can assure you a
    toymaker would look at this option first -- it's cheapest and most
    reliable.

    If you want something a little more esoteric, you might want to place
    magnets on each floor, and have a hall-effect sensor in the elevator
    (or at slightly greater expense, do the reverse). You might also have
    a separate sensor or switch for the floor (home position).

    If those don't do it, your encoder setup might be relatively simple.
    By reading quadrature, you can tell distance *and* direction. Again,
    you'll need a home sensor.

    Another possibility would be using a long threaded bolt with a riding
    nut to get the distance. If you use a metric pitch, your 5mm could be
    just a certain integral number of revolutions. Again, a switch or
    sensor to determine revolutions, and one for the home position, and
    you're done. This means gives you a great mechanical advantage, which
    can be a blessing or a curse.

    For any of this, counting revolutions may be a hassle. If so, you
    might want to use a stepper motor instead of a gearmotor -- it has a
    set number of steps per revolution, and you can just count those.
    Steppers are easy to interface to digital logic or microcontrollers

    You'll notice I'm kind of edging away from your original suggestion.
    The reason is your use of the word, "precise". Resistive tape doesn't
    exist, and even if it did, it would probably wear out right away from
    the force and friction of the wiper. But it's not very likely you'll
    be too satisfied with the precision of any resistive solution that's
    relatively inexpensive. Various solutions might include gearing to a
    potentiometer, using resistive wire element with a wiper and a current
    source. No linearity. Using these to accurately gauge distance won't
    work. Your results might be repeatable, but they definitely won't be
    accurate.

    The expensive means is to use a linear potentiometer, but they're too
    expensive for toys. Don't go there.

    Hope this has been of help. If one of these ideas sounds promising,
    and you'd like to know more, feel free to post again.

    Cheers
    Chris
     
  10. Tom Biasi

    Tom Biasi Guest

    Hey Chris,
    I like this approach.
    He could stop the motor on every tooth of a gear but override the action
    with relay shunt contacts.
    On the desired floor there could be a microswitch to drop the relay and let
    the gear stop the motor.
    I like it.

    Tom
     
  11. Guest

    I don't see anything popping up like that at Digikey. However, it looks
    like resistance wire is pretty common. Here's a couple of examples:

    Resistance Wire 1.30 ohms/foot 22 ga KAN AF
    Resistance Wire 5.23 ohms/foot 28 ga KAN AF

    http://www.google.com/search?hl=en&q="resistance+wire"
     
  12. lerameur

    lerameur Guest

    Thanks for your ideas. So far I think the best way is using a servo
    motor with roughly 3% angle shift. I do not want this to be too
    complicated and costly. I do not know how reliable are servo motors,
    nerver worked with them. Just a it worried that after many trips up
    and down it might skip a gear knotch.

    ken
     
  13. Rich Grise

    Rich Grise Guest

    On one of those home improvement shows on TeeVee the other day, they had
    made a dumbwaiter from a garage door opener.

    Good Luck!
    Rich
     
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