modulated photodetection circuit

Discussion in 'The Projects Forum' started by sdh314, Jun 2, 2011.

  1. sdh314

    Thread Starter New Member

    Jun 2, 2011
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    Hi all,

    I'm wanting to build a circuit that measures light absorption through a material. A LED is modulated in the kHz range (via a digital out line from an Arduino UNO) and a photodiode circuit measures the signal on the other side of the material i'm measuring (using a LED in the red). I then want to read in the signal with a 16 bit ADC controlled via a SPI interface. Ideally I'd like to measure both the AC (modulated) and DC components into the ADC (though recognise I might need two channels into the ADC for this)

    The ambient light intensity is likely to vary quite significantly during measurement, and I'm curious the best way to tackle the circuit design. I'll need pretty decent gain because the absorption signal isn't going to be large, but at the same time I don't want a large ambient signal to saturate the photodiode.

    I'd be hugely appreciative if anyone can through a few suggestions my way. Obviously I'm interested in a low noise solution as to take as much of an advantage of the full resolution of the ADC. I'm guessing a demodulating circuit (coupling the 0-5V square wave signal from the Arduino pin with the output from a photodiode/transimpedance amp stage) could be a good place to start? Or can what I'm trying to achieve be without the need for a synchronous circuit?

    Thanks in advance!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Do the measurement inside a light proof enclosure.
     
  3. sdh314

    Thread Starter New Member

    Jun 2, 2011
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    Unfortunately not possible, the device needs to work outside in sunlight...
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    It sounds as though you've got the basic features there already. If you can turn the LED on and off, you can AC couple the output of the photodiode, and that'll be the component of the light that comes from the LED.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    But what if the device is inside? Florescent lights have an appreciable flicker factor you will need to deal with. Even someone close to the unit may affect the amount of sunlight falling on the device while a measurement is being performed.

    I once made a sensor that used two colors of light to detect the presence of contaminants (actually a minute amount of blood) in water. I discovered the room lights would add to my readings and give me skewed data. So to take a reading I would actually take three readings: LED off, LED on, and LED off again. If readings 2 and 3 were the same I kept reading 2, if not did it again.

    The net reading was #2 - either end (or knowing me I would average them anyway). That eliminated the flicker factor. I could put my bench lamp very close to a unit and it would chug along fine.

    The difference in readings between a red and a blue LED told me if there was too much yuckie stuff in the water.

    Bottom line: however you do the ambient light subtraction you need to make sure the light at the end of the measurement is the same as it was at the beginning.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    The battle is more signal, less noise. So you need to do everything you can do to make the LED brighter. Use pulsed high current, and maybe optical focusing. All this while reducing noise as much as possible. Shade/filter out ambient light as much as possible, narrow the bandpass of the receiver to reject everything not near the modulating frequency.

    How are you going to correct for wavelength (color), or maybe that isn't important?
     
  8. someonesdad

    Senior Member

    Jul 7, 2009
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    My opinion is the lock-in is the way to go, if you have one. If not, there are a number of designs out there you can build. Dick Cappells built one for an ohmmeter he made and Linear published app note 98 about doing roughly the same thing (emulating what the HP 4328A milliohmmeter did).
     
  9. steveb

    Senior Member

    Jul 3, 2008
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    I'll second that. There is no better way to exlude noise from an optical measurement.

    Add optical filters to, to the extent that you can. This will relive the saturation issues somewhat.
     
    Last edited: Jun 2, 2011
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