Help with Optical Sensor Fluorometer

Discussion in 'Test & Measurement Forum' started by joshagirgis, Aug 27, 2018.

  1. joshagirgis

    Thread Starter New Member

    Aug 27, 2018
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    0
    Hi All, I am new to this forum.
    I am a mechanical engineer doing some ocean science and dabbling a lot in robotics, electrical engineering, and ocean engineering.

    Today I want to build an in situ fluorometer. This is a sensor which measures the fluorescence of a water column. So the sensor looks into the water and sends a blue LED light into the water. The chlorophyll in the water will fluoresce and reflect light back. This light can be measured and then correlated to the chlorophyll content.

    Here is my design
    sensors-13-07872f1.jpg
    This is going off of a paper that I read here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3715229/
    sensors-13-07872f3.jpg LED_Fluorometer.PNG
    However the design is sort of ridiculous. The transimpedance amplifier has a 5Gohm gain. When I built this I could not detect any fluorescence or differentiate from noise. I would move in my chair and the voltage output would spike like crazy just from the static i suppose.

    Here is my purposed design to filter out noise(ambient light, static).
    f circuit.PNG
    It works by using a lock-in amplifier. Modulating the light source at 2kHz using an Arduino and using the SPDT to only read the signal when the LED is "High" or 5v. Then the High pass filter filters everything below 2kHz.

    (modulating the LED using the arduino tone(pin,frequency) function)

    The problem is I can't really read any light at all. I assume this is because the filter is blocking the signal I want. Without the filter everything is really noisy.

    Are there any suggestions about what I can do for an optical Sensor?

    Thanks

    joshua
     
  2. joshagirgis

    Thread Starter New Member

    Aug 27, 2018
    11
    0
    The op amp is powered +/- 12v
     
  3. ArakelTheDragon

    Active Member

    Nov 18, 2016
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    43
    Also did you account for the homogenous characteristic of the substance in the water? The different sun angle and sensor angle? Other factors that will change the color?

    The design you have is a simple regulator circuit. The idea is like this: sensor -> regulator -> output.
     
  4. Janis59

    Active Member

    Aug 21, 2017
    579
    92
    The simplestt solution for this problem is well known. First, You get the very tiny signal, thus must to amplificate it. Yes, there are many op-amps capable to do this job, BUT all hits to Nyquist. Thus the Ukrainian nuclear physicists few years ago published a two bjt schematic working so noisless that they can count with preper pin diode the separate quantum pulses, the noise under 0,1 dB. Yet they fished out the best transistors from the large box, as normal F for their BC546 is 0,4 dB. So, now we have a signal, but for role of gates the wonderful simplicity gives the LM567. The tone generator is inside for excitator diode, the demodulator is inside, already synced, the AM output is there, the FM output is there, the Yes/No identification output is there too.
     
  5. joshagirgis

    Thread Starter New Member

    Aug 27, 2018
    11
    0
    Hi Janis59,

    Thanks for the feedback. How would one use the LM567 with this sort of sensor; is there a simple diagram I could follow for that? And is the LM567 just acting in place of the SPDT? How would I differentiate the tiny signal and amplify only that?
     
  6. joshagirgis

    Thread Starter New Member

    Aug 27, 2018
    11
    0
    Hi Arakel,

    The common in situ fluorometer sensor is a ninety degree angle between the light emitted and the light reflected. So a 45 degree angle between the LED and photodiode will do the trick. As for the homogeneous characteristic, I am not sure. I think my problem is I cannot even detect the fluorescence in the water.
     
  7. Janis59

    Active Member

    Aug 21, 2017
    579
    92
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