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Optical Feedback techniques

Discussion in 'Electronic Design' started by Charlie E., Apr 9, 2012.

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  1. Charlie E.

    Charlie E. Guest

    Hi All,
    Ok, an interesting technical question for the brain trust...

    Going for a second generation color reader, and it was suggested to me
    by a collegue that using optical feedback could greatly increase my
    accuracy. AIUI, this would involved puting a photodiode or other
    photodetector in the direct light path from my LEDs to directly
    measure their light output, and then adjust the LED output (via PWM)
    to meet a specified target output. I can think of several ways of
    accomplishing this, the simpliest being simply read the output of the
    PD with an ADC, and using a look up table or other means to set the
    PWM duty cycle, but are there other methods that would be better or
    more precise? Or use an analog method of having the two seperate (but
    equal?) photo outputs into an opamp to read the proportion between
    them? Will be awhile before I can get a board made, but wondered if
    there were some places I should be looking for suggestions...

  2. Joerg

    Joerg Guest

    That is very common with laser diodes because they age over time. So do
    LEDs but AFAIK it's only significant in most applications if you run
    them close to redline.

    While you can obtain a clean optical feedback from a laser diode via its
    back facet that is another story for LEDs. So the first question would
    IMHO be, how do you get a meaningful output reading that is not
    influenced by external light shining back into the LED? Not sure where
    your reader will be used but outdoors there's bright sunlight and all
    that. Indoors it's probably more the bling-bling people wear around thir
    wrists and fingers while holding the reader, in conjunction with bright
    light sources.

    The regulator loop is pretty trivial compared to that part of the job.
  3. Charlie E.

    Charlie E. Guest

    Don't know if my ascii art skills are up to this, but...

    ___________________ clear window
    ________ _________ heavy felt paper light barrier

    PD >| |< LED
    |< LED
    |< LED

    Basically, the three LEDs shine throught the window through a light
    sheild. The photodetector would be along the edge of the LEDs light
    cone, but in the direct path. (an alternative would be to put the PD
    at a 45 degree angle to get the primary reflection...) You take one
    reading with no LEDs on, and then readings with each individually on.
    The actual sample needs to be directly against the window for best
    accuracy. The LEDs are on a single power supply (switching) set for
    20 mA operation. I would have to PWM the enable on the power supply
    to vary the output, and filter the PD inputs.

  4. Joerg

    Joerg Guest

    Hmm, I guess this one became butchered in transmission.

    One way that's sometimes done is this:

    Window -----------------------------------------
    Paper ------------- ---------------
    \ <- V R
    \ |
    \ <- V G
    | Hole |
    V <- V B
    Photo diode - \ -
    | \ |
    45 degree Reflector

    (created by AACircuit v1.28.6 beta 04/19/05

    If the photodiode has a short black and non-reflective tube in front of
    it the chance of external light entering is miniscule. Except for stuff
    that gets reflected back off of the LEDs.
  5. Charlie E.

    Charlie E. Guest

    Yes! We are trying to get this to the lowest price possible for the
    best accuracy. Moving things, or even reflectors, or pretty much a
    no-no! I think that Joerg's idea of a black tube might be the most
    practical. I also have to worry about light reflecting off this
    calibration PD onto my main measurement PD...

  6. Joerg

    Joerg Guest

    That still doesn't fix self-pollution. Light reflected from some
    bling-bling on a finger and reflected back inside will be in phase and
    fool your detector. That's one reason for the design I've sketched.
  7. Joerg

    Joerg Guest

    Reflectors can be super simple. On one design I've helped debug we used
    snippets of thin Perspex or something like that. That way it's partly
    translucent and the cost was mere pennies. We didn't even need a hole
    that way.
  8. Charlie E.

    Charlie E. Guest

    have any links to this type of material? I could theoretically use
    this as a four way splitter, with the target PD looking at right
    angles to the calibration PD...

  9. Joerg

    Joerg Guest

    I don't remember the source, some plastics outlet. But once when somone
    accidentally jammed a screwdriver in there and busted this 45-degree
    reflector I didn't have any. I just ripped a see-through hard plastic
    insert out of a bonbon carton. Worked exactly like the original and we
    could have cut a hundred or more from that one carton.

    Of course, I had to volunteer and eat all those bonbons because the box
    and their potential freshness was now compromised :)
  10. Joerg

    Joerg Guest

    If it does then it'll be the expensive kind, the more important the
    person feels the more karat :)
  11. Jamie

    Jamie Guest

    I've taken clear acrylic, heated it in a controlled oven to soften it
    but not burn it, pushed in LED's while it was soft. afterwards, you can
    clean it up with find emery and then hit it with a heat gun which will
    then take any abrasion that you in curd to make it turn to glass looking

    I made a few optical bezels doing this and I've notice that this
    material works very well, giving a glossy glass surface.

    I've seen prisms made with that stuff just starting with a block of
    plastic, cut it, sand it smooth as possible and then hit it with a heat gun.

  12. Jasen Betts

    Jasen Betts Guest

    how about analogue feedback using an op-amp?
  13. Charlie E.

    Charlie E. Guest

    Oh, the sacrifices we do for the sake of our art... ;-)
  14. Charlie E.

    Charlie E. Guest

    Yes, I have considered using my existing plastic window in this
    fashion. My chief worry then, though, is that now you would also get
    signal from the sample as well...

    Hmmm... read with no sample, read with sample... nope, too
    complicated for the user!

  15. Joerg

    Joerg Guest

    Good idea. I guess the emphasis is on "almost non-reflective" and the
    groove helps to scatter off shallow angle light.
  16. Charlie E.

    Charlie E. Guest

    Many existing units do exactly that, but it is a step that is fraught
    with complications. Is the white target dirty? How do you signal to
    do it? Does this mean that you need additional switches, longer on
    times to give the user time to find the target, or other expensive
    options? Some units literally have a 'lens cap' to protect the front
    of the unit that is white to provide that calibration step. I want to
    make my unit a lot simplier to the user - just hold it against your
    target and press the button...

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