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More questions on color reader...

Discussion in 'Electronic Design' started by Charlie E., Apr 26, 2010.

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  1. Charlie E.

    Charlie E. Guest

    Hi All,
    You have given me good advice in the past, and I am now really close
    to shipping this thing, but still running into some of the same old

    Basically, when I program a unit, it works great here on the bench,
    and around the house, but when I go out into the real world, all heck
    breaks loose!

    My present problems seem to revolve around dark colors. Browns shift
    to dark red, or green, blacks suddenly become dark greens, dark denims
    become black, dark green, or even dark blue-green.

    Trying to determine the cause is difficult, because the problems never
    happen in the lab when I am in debug, and can get full data on what is
    going on internally. My present guesses all point to shifts in the
    strengths of the LEDs and other electronics, perhaps with temperature,
    or maybe with differences in background lighting leaking into the

    So, can anyone offer any suggestions? You can find a schematic and a
    photo of the unit at

    Thanks in advance!

  2. Joerg

    Joerg Guest

    As Jim hinted you might have background light get in. Since there is
    only one phototransistor and not three with suitable filters the unit
    can't predetermined the ambient light energies per band. At night you
    may also see errors due to mercury vapor street lighting and so on,
    worst case also some PWM modulated stuff.

    This will be worse with darker colors because the signal to noise ratio
    is lower.

    BTW, "rainbow" can mean something entirely different in places like San
    Francisco :)
  3. Charlie E.

    Charlie E. Guest

    It IS a PIc in an analog circuit! ;-)

  4. Guest

    You mean like

    "Red, Orange, Yellow, Green, Blue-Green, Blue, Purple, Pink"
    "Plus: Whitey, African-American, Gray Davis, Jerry Brown, ..."

  5. Charlie E.

    Charlie E. Guest

    Hi John,
    The upper pin is connected to ground. The VCC is on top of the bypass
    cap for the chip...

    U2C hasn't given me any problems, to this point. ;-)

    U8/U9/U10 are all complementary MOSFET pairs, doing the switching of
    the LED power to the LEDs.

    A note on background leakage. The LED and PT are both oriented to the
    front of the case. I have black felt glued to the top and bottom of
    the case (about 1" wide) to block reflections and reduce ambient. My
    typical 'CLEAR' reading (no LEDs active) is in the <0.5% range. My lab
    has a large east facing window, and I have done testing both in the
    morning, afternoon, and evening without a lot of difference. Direct
    sunlight is another matter...

  6. Joerg

    Joerg Guest

  7. Joerg

    Joerg Guest

    I think Charlie does modulate the LED already. But no matter how linear
    the phototransistor, there comes point where it can't cope with ambient.
    Especially if things are modulated.
  8. Joerg

    Joerg Guest

    Sunlight and thus outdoor ambient has a lot of IR content which goes
    through just about anything. Indoors there will be very little, AFAIK CF
    lamps emit very little in IR percentage and windows keep it out as well.

    You might need some foil in there as well, between plastic and felt.
  9. Charlie E.

    Charlie E. Guest

    Maybe a possibiliity, but the phototransistor is an ambient light
    detector, which has built in filters in the IR range to help shape the
    response curve. Pam was more concerned with possible UV contamination
    from the bright flourescents in the stores.

  10. Joerg

    Joerg Guest

    Well, it just says "photo npn" in your schematic. Which one is it? IR
    filtered doesn't necessarily mean is has to be a very good filter.
  11. Charlie E.

    Charlie E. Guest

    It is a TEPT5600, a 5mm through hole part, which I bend to a 90 degree
    angle to face the front. The LED is a Kingsbright WP154A4SUREPBGVGAW
    RGB LED which I also bend to a 90. I have the LED at a 45 degree
    angle to the face plate, and the PT and LED are on opposite sides of
    the board, to reduce direct reflections between them.

  12. Guest

  13. Joerg

    Joerg Guest

    Figure 7 hints that this photodiode still lets in a ton of the near-IR

    Might also be good to put a snippet of metal tubing around it as a
    shroud because it seems to be a plastic diode. The more you can reduce
    the acceptance angle the better.
  14. Joerg

    Joerg Guest

    Oh, they do. There was a story in our paper but it makes no sense to
    post links from there because they have obfuscated server access for
  15. Charlie E.

    Charlie E. Guest

    Interesting. It doesn't really do that much, does it... ;-)

    Ok, that is an error in the schematic. Those are all three 10K
    resistors. It was correct on the board... ;-)
    I had built in a test for the 'CLEAR' component, where I measured the
    ambient, and subtracted it from the measured values, built in testing
    it was always so low I didn't feel the need to keep it in. Might need
    to test that hypothesis now...


  16. Charlie E.

    Charlie E. Guest

    It is definitely not an intentional radiator, and no clock outputs
    leave the chip. The two switchers are only going to a cap less than
    10 mm from the chip. Would never have thought anything this simple
    could need certification. Jeorge? Any thoughts?

  17. Joerg

    Joerg Guest

    [FCC regs]

    Well, there are paths of self-certification:

    I run into this a lot. Is a change serious enough to warrant re-cert?
    Most clients do it anyhow, send the stuff to an EMC lab. But it's
    expensive, basically we rarely get out of there under $5k. If this is
    more like a non-profit product and helps the visually-impaired you may
    be able to convince a lab to do a "charity run".

    But first I'd fix that "CLEAR" ambient subtraction routine that must
    have fallen through the cracks ... whoops ... got to have that :)
  18. Charlie E.

    Charlie E. Guest

    Ok, so if we market this device in the US, since it has a MCU in it,
    then we have to get certified by the FCC that we don't radiate?

    We may just have gone out of business!!!!!!

  19. Guest

    Looks like something NASA would do.
  20. Guest

    Or a competitor gets a burr under their saddle.
    IME, EMI compliance is pretty cheap (though test labs vary quite a bit). It's
    all the rest of the stuff that takes time and $$. If there is no AC power
    (even a charger) most of that goes away.
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