help me in Photodiode sensor

Discussion in 'The Projects Forum' started by shiva, Feb 15, 2009.

  1. shiva

    Thread Starter New Member

    Feb 15, 2009
    3
    0
    Hello friends

    I am making a sensor using photodiode. photodiode's following values are changing when light from LED is passed through different specimen:

    without biasing - just when a digital mutlitmeter is connected, the resistance of the photodiode alters for every specimen. but it keeps increasing and does not stay at constant. can I use dI/dt instead?

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
  3. shiva

    Thread Starter New Member

    Feb 15, 2009
    3
    0
    Hi

    Thanks for your help. I have gone through those pages it was really useful. But still my doubts could not be solved.

    Please refer to the folowing scheme:

    [​IMG]

    For every specimen, i get changes in resistance (without biasing) but it keep changing continously. how can i fix that so that i can use it for calibrating.

    For every specimen, when biased, i get few mV - different for different specimen so that i can identify what specimen is. but this also changes every time when i change specimen and revert to earlier one.

    How can i go about this.

    Thanks friends
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I think you are reverse-biasing the photo-diode with the ohm-meter function of your multimeter then measuring its leakage current. That is one way.

    You could use the photo-diode as a solar cell that produces a very small voltage and current when light shines on it. Then the multimeter measures the voltage or current.

    If the reading changes when you revert to a previous test then there is something unstable mechanically in your test setup. Maybe different ambient lighting.
     
  5. shiva

    Thread Starter New Member

    Feb 15, 2009
    3
    0
    Hi friends,

    I made it possible by using it as a solar cell. thank you audiguru.

    Now i need some theoritical help:

    I get different current for different substances. how does current in photodiode shows the properties of photon passing out of (liquid)specimen?

    how that will relate specimen's physical property (for example, refractive index and current or molecular size and current; ion concentration and current etc) so that i can relate the liquid specimen property with the current.

    Thank you again
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The intensity of the signal is dependend on the absorbance or transmittance of the tested material.
    You are getting near the spectroscopy.
    http://www.spectroscopynow.com/coi/cda/detail.cda?id=18427&type=EducationFeature&chId=2&page=1

    The signal depends on four factors:
    1) the spectrum of the lightsource.
    2) the light that is blocked by the tested substance.
    3) the material of the substance holder.
    4) the spectral sensetivity of the light detector.

    Greetings,
    Bertus
     
Loading...