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Method to track shadow from light over a 5 cm sensor strip

Discussion in 'Sensors and Actuators' started by burt46, Jan 23, 2016.

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  1. burt46

    burt46

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    Jan 23, 2016
    Hi,

    This is my first post as i'm hoping someone could help with my specific sensor problem.

    I would like to measure the speed of a shadow as it progressively covers a sensor area. The problem is that i need at least 5 cm of distance which a photoresistor doesnt give me. I have searched the internet and not found any sort of photoresistor or photodiode with a length of 5 cm. They are all round and up to around 1 cm.

    Does anyone have an idea if there is a sensor (ideally analog) that is in a continuous strip format to allow me to measure the shadow position accurately? Open to any ideas. A simple sketch is attached of the idea.
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Why not have two sensors about 5cm apart and measure the time between one sensor falling into shadow and then the next?
     
  3. burt46

    burt46

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    Jan 23, 2016
    Thanks Steve, but I cant do that as the speed varies over the 5 cm distance (the shadow changes as the light angle changes over the 5 cm distance). With two sensor is just get the start and finish times which i can use.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Then use as many sensors as you need to track a number of points. I guess you could also use a linear ccd or even a ruler and a camera.
     
  5. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    The ones on ebay give 1mm They cost $1.00 for 20. I hope this is not too much for you.
     
  6. burt46

    burt46

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    Jan 23, 2016
    Colin, good idea, but logging 50 sensors would be a pain in the backside if i can do it with one single data value.

    I may have the answer, which i am currently testing. House a single LDR in a tube with a slit down one side that is exposed to the light/shadow. The light entering the tube through the slit will change the photo resistor at one end. Bit cumbersome, but it may just work.
     
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    The problem you've got is the fact that a bright day at the beginning is the same as a dull day over half the sensor. That's why you can't use a single sensor.
    You need a "reference."
     
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