Photodiode sensitivity vs distance

Discussion in 'The Projects Forum' started by nacoolp, Jul 11, 2013.

  1. nacoolp

    Thread Starter New Member

    Jan 2, 2013
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    Hello all,

    I am using a rotary laser light transmitter and a photo diode and its filter circuit to detect the former even in the sunlight. Now when I use the same at a distance up to 300 m the detection occurs perfectly. But beyond this distance, the diode hardly detects the light.

    Circuit diagram is given in the pdf file attached for reference.

    Thanks in advance.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Do you have a question?

    The answer is optics.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think the answer may be that the photodiodes are forward biased, and R1 is much too small. I would reverse the photodiodes, and change R1 to 100k.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Even though it already works fine at 300m ?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Maybe he can get more output by reversing the diodes, or maybe the diodes are already reversed.
    I don't have a lot of experience with photodiodes, but in the literature, I have never seen a photodiode being used in the forward-biased mode.
     
  6. nacoolp

    Thread Starter New Member

    Jan 2, 2013
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    Thanks Wayneh Ron H for your feedback.

    And Sorry for a big mistake.

    The photodiodes are used in the reverse biased mode only. By mistake its orientation was reversed.

    Thanks and enlighten me with your view when the diodes are reverse biased.
     
  7. Ron H

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    Apr 14, 2005
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    What are your frequency response requirements (low and high frequency corners)?
     
    Last edited: Jul 12, 2013
  8. nacoolp

    Thread Starter New Member

    Jan 2, 2013
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    My frequency response requirement is any signal above 3 Hz (to nullify the sunlight).

    I have tried a lot of different combinations for the same but to no avail. Now I am confused which of the following could be the factor responsible for this scenario:

    1) Capacitor C1 (4.7nF) is not discharged completely as with the increasing distance, contact time between laser light and photo diode reduces.

    2) Low gain of transistor as with increasing distance the intensity of light reduces to a certain extent and thereby weakening the signal.

    Thanks and Regards
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    1. The charge on the cap should not change appreciably.
    2. I don't understand what you mean.

    The circuit you posted has a low frequency corner of ≈15kHz. To get ≈3Hz, change R1 to 100k and C1 to 470nF.
     
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