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Counting The Spots

Discussion in 'Electronic Basics' started by Ron Hubbard, Feb 19, 2008.

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  1. Ron Hubbard

    Ron Hubbard Guest

    I just bought a new strobe light for an experiment in biofeedback;
    supposedly it can flash up to 10 fps but I need to know when or if
    it's flashing at pretty much 7 fps. Does anyone know if there is any
    quick and dirty way to determine the flash rate?

    Ron
     
  2. Tim Wescott

    Tim Wescott Guest

    Hang it on a rope long enough for a 1 second period, and do like your
    title says -- count the spots.

    I think you want around a one meter rope, but it may need to be four --
    do a web search on 'pendulum' & you should find the right number.

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  3. Bob Monsen

    Bob Monsen Guest


    Video camera filming it and a watch. View it on slow speed. Count the
    flashes...

    Regards,
    Bob Monsen
     
  4. Phil Allison

    Phil Allison Guest

    "Ron Hubbard"

    ** Make a pendulum with a length of cotton tied to a small metal nut.

    Set it so there is 20 mm from the fulcrum to the centre of the nut.

    The period will be 0.28 seconds or 0.14 seconds between peak excursions.

    A 7 Hz flash should be able to stop motion at each peak excursion.



    ....... Phil
     
  5. Eeyore

    Eeyore Guest

    A nice practical application of basic physics there Phil. :~)

    Graham
     
  6. Phil Allison

    Phil Allison Guest

  7. Ron Hubbard

    Ron Hubbard Guest

    Hmmm, i was hoping for something more... electronic...

    Ron
     
  8. Phil Allison

    Phil Allison Guest

    "Ron L. Hubbard"

    Hmmm, i was hoping for something more... electronic...



    ** Depends what test gear YOU have available.

    Got a scope ?

    Got anything?



    ........ Phil
     
  9. Ron Hubbard

    Ron Hubbard Guest

  10. Bob Masta

    Bob Masta Guest

    Can you rig up a phototransistor to convert the light pulses to
    voltage pulses? Then you can use a frequency counter
    to measure the rate. Hopefully, your counter has a Period
    mode, so you can get an update on every flash, and not have
    to wait many seconds for decent resolution. You just have to
    know that 7 Hz = 142.857... msec.

    No counter? You can use your sound card with the built-in frequency
    counter in Daqarta. It does the period-to-frequency conversion
    auotomatically, and you can enlarge the display to fill your screen if
    you want. (For personal/hobby use, Daqarta is US$29 for a lifetime
    license, but you can try it for 30 sessions / 30 days for free. After
    that it won't accept input signals, but the signal generator and
    analysis functions continue to work forever. If you can get all your
    measurements done in the trial period, you are all set!)

    Best regards,


    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
     
  11. Tim Wescott

    Tim Wescott Guest

    Cotton string is _filled_ with electrons. Without them, it'd fly apart
    with a pretty impressive bang.

    You wanted to know if it was going "pretty much" at 7Hz. Phil gave you a
    solution that's better than mine, right down to the fact that he
    specified the length of the pendulum (although both of us neglected to
    stipulate that you have to change the length of the pendulum if you
    change planets).

    You could do this with a power supply, and a resistor and a pin diode or
    phototransistor and an oscilloscope -- but isn't it easier to tie a nut
    on a string?

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  12. John Larkin

    John Larkin Guest

    Do you still have a record player? Put a dot on the turntable and
    flash it with your strobe. Do some math.

    John
     
  13. Chris W

    Chris W Guest

    You should really consider the pendulum idea. The time it takes a
    pendulum to swing through one cycle is VERY consistent. That is why
    most mechanical time pieces use either a gravity or spring activated
    pendulum. It shouldn't be too hard to calibrate to some speed that will
    work for what you are doing. To make it easy just make the period 7
    times what the period of the strobe needs to be, an even one second. To
    calibrate it, simply use a stop watch to time 20 or more cycles then
    shorten the string to be faster and lengthen it to be slower. Once you
    have its cycle set to that time, then adjust your strobe light till you
    see 7 stationary images of your pendulum. To minimize loss in the swing
    have the pivot be something very smooth and use light weight string tied
    loosely around the pivot and use a heavy smooth object at the bottom of
    the string (steel ball). That way it will swing for a longer time on
    it's own before you need to reset it.


    --
    Chris W
    KE5GIX

    "Protect your digital freedom and privacy, eliminate DRM,
    learn more at http://www.defectivebydesign.org/what_is_drm"

    Ham Radio Repeater Database.
    http://hrrdb.com
     
  14. Chris W

    Chris W Guest

    A point I forgot to make to illustrate this point, make a pendulum and
    start it's swing at say 60 degrees from vertical, time 10 cycles. Then
    do it again starting it's swing from 20 degrees. Even though it is
    moving a much shorter distance in the second test, you will see that the
    speed is nearly exactly the same. If you do it in a vacuum, with a zero
    friction pivot, it will be EXACTLY the same. Even with out the vacuum
    the friction from the air and a good pivot will be negligible at the
    speed it will be swinging.

    --
    Chris W
    KE5GIX

    "Protect your digital freedom and privacy, eliminate DRM,
    learn more at http://www.defectivebydesign.org/what_is_drm"

    Ham Radio Repeater Database.
    http://hrrdb.com
     
  15. Tim Wescott

    Tim Wescott Guest

    1. Yes, I am picking nits.

    2. No it won't. The period gets longer as the angle gets greater and
    the apparent torque on the pendulum gets smaller than a linear
    projection of sin(angle). The difference is very slight, so only
    clock wonks pay any attention to this.

    3. Even if you're a clock wonk all is not lost. In order to make a
    pendulum's period independent of the magnitude of the swing you
    can modify the pivot so that the center of mass of the pendulum
    travels in a parabola, not in an arc of a circle. This can be
    done by suspending the pendulum from a thin strip of metal that
    is constrained to wrap around a pivot with just the right
    curvature (a 'cycloid', IIRC).

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  16. Chris W

    Chris W Guest

    Hmm, I don't remember that little detail from my engineering physics
    class. Of course that just means, either we ignored it, or I forgot it.
    I do remember that in our lab experiment, I could not detect a
    difference in cycle time regardless of the angle using a hand stop watch
    and timing enough cycles for it to take between 30 and 60 seconds.

    I'm not sure what you mean by the apparent torque on the pendulum gets
    smaller with greater angles? Do you mean the torque on the pendulum
    pivot? If so, why would that not get greater with a greater angle? I
    am considering 0 degrees to be the angle with the pendulum at rest.

    Wait I just read that again and I think I know what you are trying to
    say... Are you saying that with larger angles the actual "apparent
    torque" will be farther from that calculated by projecting the vertical
    gravity vector onto the tangent vector of the pendulum? If I am
    understanding you correctly, can you tell me why that is?

    Given the similarity of the equation for distance over time, at a given
    acceleration, and the equation of a parabola, that explanation does, as
    my calculus teacher liked to say, give you a warm fuzzy feeling.



    --
    Chris W
    KE5GIX

    "Protect your digital freedom and privacy, eliminate DRM,
    learn more at http://www.defectivebydesign.org/what_is_drm"

    Ham Radio Repeater Database.
    http://hrrdb.com
     
  17. Tim Wescott

    Tim Wescott Guest

    (snip)

    To have a perfectly consistent period the torque on the pendulum would
    have to go as k * theta, with theta being the pendulum angle and k being
    some constant (with a free hanging pendulum k is determined by the
    pendulum length and acceleration due to gravity).

    But for a pendulum that's suspended at a point, the torque on the
    pendulum goes as k * sin(theta). Sin(theta) is close to theta for small
    angles, but |sin(theta)| < |theta| for all theta != 0, so there you are.

    This won't start to make a difference until you have cleaned up a lot of
    other sources of error, so for your first clock-building project I would
    suggest using a simple pivot.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  18. Ron Hubbard

    Ron Hubbard Guest

    Yep, I do have an old Heathkit 'scope; very basic, nothing fancy. I
    also have-- somewhere-- a multimeter. That has a frequency counter,
    but it's unreliable below 20 Hz. I had always meant to make a good
    frequency counter, but I never got around to it. ;-(

    Ron
     
  19. Rich Grise

    Rich Grise Guest

    If the scope is well-calibrated, then all you need is some kind of
    photodiode or something and a battery. Actually, if you had a calibrated
    marker oscillator, you wouldn't need the scope to be that well-calibrated.
    But if you had some bacon, you could have bacon and eggs, if you had
    some eggs. ;-)

    But why 7 Hz? Are you trying to get seizures? Or maybe spoof those
    emergency vehicle things that turn your light green and all the rest
    of them red? ;-)

    Good Luck!
    Rich
     
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