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electronic stopwatch "speedometer"

Discussion in 'Electronic Design' started by Smitty Two, Oct 21, 2009.

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  1. Smitty Two

    Smitty Two Guest

    I'd like to make a relatively inexpensive stand-alone gizmo to measure
    the speed of a steel ball on a track as an accessory for a kid science
    experiment. So I'm thinking that an emitter-detector straddles the
    track, and the ball interrupts the beam.

    The associated circuit sends a start pulse to an electronic timer when
    the detector goes dark, and a stop pulse to the timer when the detector
    sees light again. The little rug rats can then calculate the velocity of
    the ball based on how long the pulse was, knowing the ball's diameter.

    But, I don't know what to look for in terms of a little digital panel
    meter that hopefully has some sort of timer function built-in. Any leads
    would be greatly appreciated. Either the source of such a device, or at
    least a buzzword to google for.
  2. Smitty Two

    Smitty Two Guest

    All right then. Cruising eBay I see a lot of cheap stopwatches, but most
    of them are 1/100 second. I'll likely need 1/1000, but I found a couple
    of those in the $25 range which isn't a deal breaker. Thanks for the
  3. Rich Grise

    Rich Grise Guest

    With something like that, it might be a good idea to get the more advanced
    kids (what age range?) involved in building the kit, and trying to
    understand the circuit.

  4. Rich Grise

    Rich Grise Guest

  5. Rich Grise

    Rich Grise Guest

    Or use a digital oscilloscope and get the kids interested in that,

    When I was a kid I measured the velocity of a .22 rifle, breaking two
    wires and using an analog scope. It's tricky to aim the rifle while
    looking at the scope. I used a stack of old Tom Swift books to catch
    the bullet.

    "There's more than one way to skin a cat," said Tom categorically.

  6. Smitty Two

    Smitty Two Guest

    Agreed. I'm not sure what the velocity of the ball is. In the range of
    1m/s I would guess, and the diameter is 1/2", so it's true that a
    millisecond timer isn't going to give me as much resolution as I'd like,
    but hopefully it's close enough. The dual-sensor approach is good in
    that regard but of course it adds to the parts count.
  7. Smitty Two

    Smitty Two Guest

    Well, this thing* started out as a fun way to pass an hour in the
    evening, and then the kids took it to their sixth grade science class,
    where it was a hit. So I'm going to donate it to the school for future
    classes. The speedometer is just an enhancement, but I don't want to
    spring for an o'scope.

  8. Smitty Two

    Smitty Two Guest

    Thanks Doug. Good thing someone else replied to you or I wouldn't have
    seen your post, since I block googlers by default. But anyway, I think I
    had found that thing in my searching, and ruled it out as too expensive.
    Looks like it'd be $25 for the main kit and another $15 for the
    stopwatch module. Now that I look into it further I see it only has
    resolution of 0.01 second, also.
  9. Smitty Two

    Smitty Two Guest

    Thanks, Jim. Not sure about the hall effect sensors, I've never used
    them and my understanding of them is weak. Can't magnetize the balls,
    it'd be counterproductive for this application (magnetically driven
    linear accelerator)
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