photocell

Discussion in 'General Electronics Chat' started by amir ee, Apr 4, 2008.

  1. amir ee

    Thread Starter New Member

    Apr 4, 2008
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    I need professional information about photocell...
    or if you can't help me,please tell me which sensors are suitable to apecify the light beams path?
    Thank you...
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    You may be asking about LDR's (light dependent resistors). There are also phototransistors and integrated devices with photodiodes and amplifiers.

    Can you tell us more about your application?
     
  3. rwmoekoe

    Active Member

    Mar 1, 2007
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    do you mean that you want to detect the path of a light beam as to which direction it is beaming?
     
  4. amir ee

    Thread Starter New Member

    Apr 4, 2008
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    a kind of sensors that can be used to measure the light intense and they are suitable to specipy the light beams direction too...

    and YES rwmoekoe!
     
  5. rwmoekoe

    Active Member

    Mar 1, 2007
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    it's a challenging one. can anybody help? or can we brainstorm about this?
     
  6. rwmoekoe

    Active Member

    Mar 1, 2007
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    will it matter if the degree of direction measured is only - say 90° conical or so, and the light beam will be somewhat reduced going through the device?
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    The mechanical design could get elaborate, but making an array of photosensors with limited lines of sight can be done. Going beyond eight might get expensive.

    It should also be possible to arrange a spinning photosensor with something like a servo pot or position encoder to indicate the direction light was coming from.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Going a bit further with beenthere's concept...

    Consider a phototransistor mounted on the edge of a spinning disk. If that spinning disk were mounted on the edge of another disk slowly rotating disk, slightly less than the diameter of the first disk, the path of the single phototransistor would be much like that of a Spirograph design.
    http://en.wikipedia.org/wiki/Spirograph

    Adjusting the relative speeds of the two disks could give very complete coverage of the swept area. The position of the reading could be interpolated from the reported positions of the two disks.

    This concept would require a somewhat complex slip-ring arrangement for the phototransistor, motors, and position encoders' signals.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Or you could arrange to sweep 360 degrees clockwise and then 360 degrees counterclockwise and repeat - no slip rings needed.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, that's a definite possibility - or just 180° either way, for both sweeps - or perhaps even less. Some of that would be dependent on the OP's need for speed of acquisition, as naturally it will take time to deaccelerate, reverse direction and accelerate the sensor. There are potentially a great many data points to be taken and compared.

    It wouldn't be too difficult to complicate this project to the bleeding edge of comprehension. :confused: :D

    However, having the limits of rotation would eliminate some potentially very useful scan patterns such as the hypotrochoid known informally as an 8 leafed rose. There is an animation of this pattern (and many more) near the bottom of this page: http://xahlee.org/SpecialPlaneCurves_dir/Hypotrochoid_dir/hypotrochoid.html
     
  11. rwmoekoe

    Active Member

    Mar 1, 2007
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    wow, great ideas!

    i've just thought about this one,
    the light beam should pass through two partially transparent mirrors consecutively.
    each mirror partially reflect the beam towards their own ccd sensors.
    we can then get the two coordinates from the two ccd's to determine the direction. (and the intensity).
    the drawback is that the light would be filtered twice and thus twice reducing its intensity.

    amir, what do you think about those ideas? i'm curious here :)
     
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