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From: dagmargoodboat on 6 May 2010 01:46
On May 3, 12:43 pm, Charlie E. <edmond...(a)ieee.org> wrote:
> On Mon, 26 Apr 2010 15:07:02 -0700, Charlie E. <edmond...(a)ieee.org>
> wrote:
> Well, it looks like it is time to call it a day.  I spent two days
> this weekend testing, and calibrating four units.  On the bench, they
> all worked great, and gave good results across my entire test samples.
>
> This morning, I mounted them in their final cases, and hooked them up.
> Two failed immediately, basically decided everything I tested was
> white.  Two appeared to function, but as soon as I started testing,
> failed on every 'corner' case in my test samples.  Took one of those
> back to the bench, and the calibrations had shifted drastically. Funny
> thing was, the shift was to needing more gain, not less, which the
> 'all white' indications would have indicated.
>
> Technically, I have been 'measuring' gain as the setting on the
> digital pot that gave an almost full indications on the ADC.  This
> gave me values from 0 to 255.  When I measured this unit on Saturday,
> it had gains of red 239, green 239 and blue 226.  On Sunday, when I
> finallized the program, it read 231, 233, 214.  This morning, after
> retesting, it calibrates at 245, 241, and 231.  So, a shift of over 5%
> in just two days.  There might have been temperature or background
> variations, but the background measuremnts have been stable at a
> reading of around 8 - 10 on a scale of 2048.  I am totally baffled!
>
> So, after a year and about $2000 in materials, looks we are going to
> forget this product, unless some of ya'll have any ideas.  

1rst, like Joerg, I'm not happy that IR is being excluded from the
detector. Many optically opaque, even black materials are like glass
to IR. Use aluminum foil, then you'll know for sure.

The phototransistor filter is, well, pathetic, not nearly good
enough. Consider:

The sun produces 1KW/m^2. That's 1mW/mm^2.

Just wildly ballparking with very generous assumptions, your LEDs, at
20mA, produce possibly

30% x 20mA x 3v = 18mW over a 10mm spot.

That's
18mW / 76mm^2 = 230uW/mm^2 _radiated_; the reflected signal you
receive will be an order of magnitude weaker.

So the fact that your sensor's filter is 75% down by 900nm is
basically no protection from sunlight at all--sunlight could easily
produce 10x more response than your signal and swamp your sensor, if
you let it in.


2nd, simplify the problem. For example,
o indirectly couple one LED to the phototransistor, attenuated,
totally excluding external light, and see if the readings drift. Heat
and cool things too.
o Measure the actual voltage at the phototransistor's emitter when
you do this--see how close to the margins it's getting pushed.
o Put the whole thing in a dark room with a controlled light
source, if need be, then try turning on an interfering light source,
like a fluorescent light, then an incandescent.
o Check the raw readings and corrected--see whether the channels
respond the same ways. (For example, your phototransistor's response
to blue light is down 70%--nearly as far as for 900nm IR--so the blue
channel might be more susceptible to the effects of stray light.)

Poke around, think, explore. You'll find it, learn something in the
doing, and your product will sing.

> Anyone out there know of anyone needing a good applications engineer?

You *can* fix it Charlie!

--
Cheers,
James Arthur
From: Joerg on 6 May 2010 10:49
dagmargoodboat(a)yahoo.com wrote:
> On May 3, 12:43 pm, Charlie E. <edmond...(a)ieee.org> wrote:
>> On Mon, 26 Apr 2010 15:07:02 -0700, Charlie E. <edmond...(a)ieee.org>
>> wrote:
>> Well, it looks like it is time to call it a day. I spent two days
>> this weekend testing, and calibrating four units. On the bench, they
>> all worked great, and gave good results across my entire test samples.
>>
>> This morning, I mounted them in their final cases, and hooked them up.
>> Two failed immediately, basically decided everything I tested was
>> white. Two appeared to function, but as soon as I started testing,
>> failed on every 'corner' case in my test samples. Took one of those
>> back to the bench, and the calibrations had shifted drastically. Funny
>> thing was, the shift was to needing more gain, not less, which the
>> 'all white' indications would have indicated.
>>
>> Technically, I have been 'measuring' gain as the setting on the
>> digital pot that gave an almost full indications on the ADC. This
>> gave me values from 0 to 255. When I measured this unit on Saturday,
>> it had gains of red 239, green 239 and blue 226. On Sunday, when I
>> finallized the program, it read 231, 233, 214. This morning, after
>> retesting, it calibrates at 245, 241, and 231. So, a shift of over 5%
>> in just two days. There might have been temperature or background
>> variations, but the background measuremnts have been stable at a
>> reading of around 8 - 10 on a scale of 2048. I am totally baffled!
>>
>> So, after a year and about $2000 in materials, looks we are going to
>> forget this product, unless some of ya'll have any ideas.
>
> 1rst, like Joerg, I'm not happy that IR is being excluded from the
> detector. Many optically opaque, even black materials are like glass
> to IR. Use aluminum foil, then you'll know for sure.
>
> The phototransistor filter is, well, pathetic, not nearly good
> enough. Consider:
>
> The sun produces 1KW/m^2. That's 1mW/mm^2.
>
> Just wildly ballparking with very generous assumptions, your LEDs, at
> 20mA, produce possibly
>
> 30% x 20mA x 3v = 18mW over a 10mm spot.
>
> That's
> 18mW / 76mm^2 = 230uW/mm^2 _radiated_; the reflected signal you
> receive will be an order of magnitude weaker.
>
> So the fact that your sensor's filter is 75% down by 900nm is
> basically no protection from sunlight at all--sunlight could easily
> produce 10x more response than your signal and swamp your sensor, if
> you let it in.
>

Yup, got to try foil and stuff. Plastic is (mostly) not very good at
muffling IR. The fact that Charlie is seeing drift that "sticks", IOW
that doesn't come back after cooling off, tells me that the budget above
may have to get worse. He may have to back off on the LED excitation
current. Maybe to 10mA or so.

[...]

>
> You *can* fix it Charlie!
>

I'll second that. With enough determination I am sure he can solve this
and leave others who possibly don't know about the IR effects in the dust.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: dagmargoodboat on 6 May 2010 11:07
On May 6, 9:49 am, Joerg <inva...(a)invalid.invalid> wrote:
> dagmargoodb...(a)yahoo.com wrote:
> > On May 3, 12:43 pm, Charlie E. <edmond...(a)ieee.org> wrote:
> >> On Mon, 26 Apr 2010 15:07:02 -0700, Charlie E. <edmond...(a)ieee.org>
> >> wrote:
> >> Well, it looks like it is time to call it a day.  I spent two days
> >> this weekend testing, and calibrating four units.  On the bench, they
> >> all worked great, and gave good results across my entire test samples.
>
> >> This morning, I mounted them in their final cases, and hooked them up.
> >> Two failed immediately, basically decided everything I tested was
> >> white.  Two appeared to function, but as soon as I started testing,
> >> failed on every 'corner' case in my test samples.  Took one of those
> >> back to the bench, and the calibrations had shifted drastically. Funny
> >> thing was, the shift was to needing more gain, not less, which the
> >> 'all white' indications would have indicated.
>
> >> Technically, I have been 'measuring' gain as the setting on the
> >> digital pot that gave an almost full indications on the ADC.  This
> >> gave me values from 0 to 255.  When I measured this unit on Saturday,
> >> it had gains of red 239, green 239 and blue 226.  On Sunday, when I
> >> finallized the program, it read 231, 233, 214.  This morning, after
> >> retesting, it calibrates at 245, 241, and 231.  So, a shift of over 5%
> >> in just two days.  There might have been temperature or background
> >> variations, but the background measuremnts have been stable at a
> >> reading of around 8 - 10 on a scale of 2048.  I am totally baffled!
>
> >> So, after a year and about $2000 in materials, looks we are going to
> >> forget this product, unless some of ya'll have any ideas.  
>
> > 1rst, like Joerg, I'm not happy that IR is being excluded from the
> > detector.  Many optically opaque, even black materials are like glass
> > to IR.  Use aluminum foil, then you'll know for sure.
>
> > The phototransistor filter is, well, pathetic, not nearly good
> > enough.  Consider:
>
> >   The sun produces 1KW/m^2.  That's 1mW/mm^2.
>
> > Just wildly ballparking with very generous assumptions, your LEDs, at
> > 20mA, produce possibly
>
> >    30% x 20mA x 3v = 18mW over a 10mm spot.
>
> > That's
> >    18mW / 76mm^2 = 230uW/mm^2 _radiated_; the reflected signal you
> > receive will be an order of magnitude weaker.
>
> > So the fact that your sensor's filter is 75% down by 900nm is
> > basically no protection from sunlight at all--sunlight could easily
> > produce 10x more response than your signal and swamp your sensor, if
> > you let it in.
>
> Yup, got to try foil and stuff. Plastic is (mostly) not very good at
> muffling IR. The fact that Charlie is seeing drift that "sticks", IOW
> that doesn't come back after cooling off, tells me that the budget above
> may have to get worse. He may have to back off on the LED excitation
> current. Maybe to 10mA or so.

RFI's a possible too. I'm skeptical of LED aging at 20mA, but that's
easily tested.

--
Cheers,
James Arthur
From: Joerg on 6 May 2010 11:55
dagmargoodboat(a)yahoo.com wrote:
> On May 6, 9:49 am, Joerg <inva...(a)invalid.invalid> wrote:
>> dagmargoodb...(a)yahoo.com wrote:
>>> On May 3, 12:43 pm, Charlie E. <edmond...(a)ieee.org> wrote:
>>>> On Mon, 26 Apr 2010 15:07:02 -0700, Charlie E. <edmond...(a)ieee.org>
>>>> wrote:
>>>> Well, it looks like it is time to call it a day. I spent two days
>>>> this weekend testing, and calibrating four units. On the bench, they
>>>> all worked great, and gave good results across my entire test samples.
>>>> This morning, I mounted them in their final cases, and hooked them up.
>>>> Two failed immediately, basically decided everything I tested was
>>>> white. Two appeared to function, but as soon as I started testing,
>>>> failed on every 'corner' case in my test samples. Took one of those
>>>> back to the bench, and the calibrations had shifted drastically. Funny
>>>> thing was, the shift was to needing more gain, not less, which the
>>>> 'all white' indications would have indicated.
>>>> Technically, I have been 'measuring' gain as the setting on the
>>>> digital pot that gave an almost full indications on the ADC. This
>>>> gave me values from 0 to 255. When I measured this unit on Saturday,
>>>> it had gains of red 239, green 239 and blue 226. On Sunday, when I
>>>> finallized the program, it read 231, 233, 214. This morning, after
>>>> retesting, it calibrates at 245, 241, and 231. So, a shift of over 5%
>>>> in just two days. There might have been temperature or background
>>>> variations, but the background measuremnts have been stable at a
>>>> reading of around 8 - 10 on a scale of 2048. I am totally baffled!
>>>> So, after a year and about $2000 in materials, looks we are going to
>>>> forget this product, unless some of ya'll have any ideas.
>>> 1rst, like Joerg, I'm not happy that IR is being excluded from the
>>> detector. Many optically opaque, even black materials are like glass
>>> to IR. Use aluminum foil, then you'll know for sure.
>>> The phototransistor filter is, well, pathetic, not nearly good
>>> enough. Consider:
>>> The sun produces 1KW/m^2. That's 1mW/mm^2.
>>> Just wildly ballparking with very generous assumptions, your LEDs, at
>>> 20mA, produce possibly
>>> 30% x 20mA x 3v = 18mW over a 10mm spot.
>>> That's
>>> 18mW / 76mm^2 = 230uW/mm^2 _radiated_; the reflected signal you
>>> receive will be an order of magnitude weaker.
>>> So the fact that your sensor's filter is 75% down by 900nm is
>>> basically no protection from sunlight at all--sunlight could easily
>>> produce 10x more response than your signal and swamp your sensor, if
>>> you let it in.
>> Yup, got to try foil and stuff. Plastic is (mostly) not very good at
>> muffling IR. The fact that Charlie is seeing drift that "sticks", IOW
>> that doesn't come back after cooling off, tells me that the budget above
>> may have to get worse. He may have to back off on the LED excitation
>> current. Maybe to 10mA or so.
>
> RFI's a possible too. I'm skeptical of LED aging at 20mA, but that's
> easily tested.
>

True, RFI can do this but it wouldn't "stick".

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
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Prev: Financial Management Theory & Practice by Eugene F. Brigham, Michael C. Ehrhardt 13th Edition 2011 Solution manual is available for purchase at affordable prices. Contact me at alltestbanks11[at]gmail.com to buy it today. All emails will be answer
Next: Any insight for Automotive Sensor Inputs/ Front-End Protection.