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More questions on color reader...

S

Spehro Pefhany

Jan 1, 1970
0
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.

Just for kicks, try hanging a couple of 10n caps from output to
inverting input on those MCP op-amps. (between pins 1/2 and 7/6).
It's possible they're oscillating, particularly the first one.
 
J

Joerg

Jan 1, 1970
0
Charlie said:
Thanks, Jeorg,
I am driving the LEDs at 20mA, which might be the problem if there are
spikes that drive it above limits. I did think to turn the power off
before switching. I even turned one LED on, then turned the other off
for a while, to be sure there were not any no load conditions on the
switcher. ...


How much pause time are we talking here, from turn-off to mux change,
and from mux change to turn-on?

LEDs operated near their mux can noticeably deteriorate, and fast. It
also depends on where they came from.

... I don't have an o-scope at this time. It was on my list
for purchases as soon as we made some sales... ;-)

A client of mine didn't either because they are all MEs. So I convinced
them to at least get a 40-60MHz DSO and they bought one from Instek for
under $500 (new), because I told them I had the big Instek and was
happy. They were amazed at what they could do with this thing. Non-EE
stuff they never even thought about, like measuring reaction times of
mechanical actuators, hydraulic stuff and whatnot. "How do we measure
closure time on this thing here?" ... "Just use that scope with both
channels on and a piezo" ... "Oh!"

I wish I was an analog guy, but this project has shown me how far I
have to go to really consider myself to have the necessary experience
to be one!

We all had to get there, and most of us on our own. Don't give up, this
looks like a good project to hone your skills and if you get it licked
it can even make you some money.
 
C

Charlie E.

Jan 1, 1970
0
How much pause time are we talking here, from turn-off to mux change,
and from mux change to turn-on?

LEDs operated near their mux can noticeably deteriorate, and fast. It
also depends on where they came from.



A client of mine didn't either because they are all MEs. So I convinced
them to at least get a 40-60MHz DSO and they bought one from Instek for
under $500 (new), because I told them I had the big Instek and was
happy. They were amazed at what they could do with this thing. Non-EE
stuff they never even thought about, like measuring reaction times of
mechanical actuators, hydraulic stuff and whatnot. "How do we measure
closure time on this thing here?" ... "Just use that scope with both
channels on and a piezo" ... "Oh!"



We all had to get there, and most of us on our own. Don't give up, this
looks like a good project to hone your skills and if you get it licked
it can even make you some money.

On the turn on/turn off times, I use a nop timer with 64K counts on a
20MHz clock, so 3 to 10 ms. I start the LED, wait, start the power
supply, wait, and then measure four times and divide by two. Turn off
the power, wait, and start over...

Charlie
 
J

Joerg

Jan 1, 1970
0
Charlie said:
Charlie E. wrote:
[...]
How much pause time are we talking here, from turn-off to mux change,
and from mux change to turn-on?

LEDs operated near their mux can noticeably deteriorate, and fast. It
also depends on where they came from.
[...]

On the turn on/turn off times, I use a nop timer with 64K counts on a
20MHz clock, so 3 to 10 ms. I start the LED, wait, start the power
supply, wait, and then measure four times and divide by two. Turn off
the power, wait, and start over...

That's done very well. This pretty much leaves age-drift of the LEDs
themselves, maybe they are just pushed too hard. Possibly only one of
them is. A way to test that would be to equip the unit that has crept up
past the 240 range with a fresh battery and see if it comes back down.

Can you ease off a bit to, say, half the current and see if the longterm
stability improves? It would be sad to throw in the towel at the last
mile. Any chance to borrow a digital scope?
 
J

JosephKK

Jan 1, 1970
0
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.

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

Charlie

Please look for my PM.
 
J

Joerg

Jan 1, 1970
0
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.
 
J

Joerg

Jan 1, 1970
0
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".
 
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