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PICList Thread
'PIC light sensitivity'
1996\12\04@192122 by D. L. Davis

picon face
I built an altimeter with a PIC14000 which I have in bench test now.  I am
using the 16 bit A/D of the PIC14000 to measure voltage from an air pressure
sensor.  It is used to control an autopilot, and the PIC runs the autopilot
servo too.

Everything works great with one big exception.... the PIC is extraordinarily
sensitive to light!  The logic all functions fine and is unaffected by
light, but the A/D measurement is strongly influenced by ambient light on
the chip.  I'm using a JW part now, with tape over the window.  But even
with tape over the window, I can effect the A/D count by an order of
magnitude just by shining a flashlight on the PIC from across the room.  I
have replaced the little red sticker tape that came with the part from
Microchip with double layers of black tape, which improves the A/D
stability, but only to about 6 or 7 bits, nowhere close to the 16 bit
resolution.  If I control ambient light carefully, it still moves around
randomly in the least significant 9 or 10 bits.

I have noticed that the chip measures its own internal reference with good
accuracy and stability and is nearly insensitive to ambient light (stable to
about 14 bits).  This much works great, but the measurement of external
voltages at the pins is hugely affected by light???? (Yes I have checked
that the voltages at the pins are stable).

I am hoping that the OTP version without a window will not have these
problems.  But I am concerned that I will not even be able to get a good
test done with the JW part.  I haven't yet found a way to get more than
about 7 bits of stability (much less accuracy) out of this JW part, which is
totally inadequate for an altimeter or autopilot application.

I have heard of EPROMS having light sensitivity, but the program and data
memory in this PIC seems to work just fine.  Only the A/D is affected.  Has
anyone else seen this light sensitivity problem?  Has anyone used this part
successfully with stability of 12 or more bits?  Any ideas out there?

                                       Dewey Davis

1996\12\04@220231 by optoeng

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D. L. Davis wrote:
{Quote hidden}

This doesn't surprise me at all.  Silicon is photoconductive, regardless
of whether you are talking about junctions or MOS capacitors (i.e.,
gates).  The degree to which a given function is affected by light would
depend on several factors:

1. Metallization: the Al layer is opaque.
2. Impedance: lower impedance circuits are less affected by leakage
3. Location: circuits nearer the center of the die have a wider field of
view

Whether the A/D is the charge balancing type, current sources, or
resistor ladders, it's bound to have some relatively high impedance
sections.

BTW, ordinary black electrical tape is not opaque to near IR.  The old
silver write-protect labels for 5.25" floppies work well, or, you can
get 3M opaque black crepe tape or something similar.
--

Paul Mathews, consulting engineer
AEngineering Co.
spam_OUToptoengTakeThisOuTspamwhidbey.com
non-contact sensing and optoelectronics specialists

1996\12\05@093748 by Craig Knotts

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    The analog portions of the circuit will be affected more noticably
    than the digital portions.  The digital circuitry is typically driven
    to saturation, masking the problem so that the slight increase in
    conductivity doesn't cause mis-operation of the digital portions of
    the circuit.

    The analog portion, however, will be affected much more by changes in
    conductivity.

    Metal tape, or labels specifically designed for EPROM window covers,
    will do a better job.




______________________________ Reply Separator _________________________________
Subject: PIC light sensitivity
<snip>

1996\12\05@103235 by Jerry Ethridge

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{Quote hidden}

Keep in mind that if you are using a silicon based pressure sensor,
it will also be sensitive to light. I have seen several altimiters
built for model rockets which had to use light sheilding on the
pressure sensor.

Just one more thing to watch out for!

Jerry

1996\12\05@181637 by Tony Matthews

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Jerry Ethridge wrote:
{Quote hidden}

In a robot sensor experiment I learned practically nothing on or around
my bench could totally block the IR emission's from a simple tv remote.
not to mention the sun. Metal worked. Tony M.

1996\12\05@203219 by hoss karoly

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D. L. Davis wrote:
>
> I built an altimeter with a PIC14000 which I have in bench test now.  I am
> using the 16 bit A/D of the PIC14000 to measure voltage from an air pressure
> sensor.  It is used to control an autopilot, and the PIC runs the autopilot
> servo too.

> I have noticed that the chip measures its own internal reference with good
> accuracy and stability and is nearly insensitive to ambient light (stable to
> about 14 bits).  This much works great, but the measurement of external
> voltages at the pins is hugely affected by light???? (Yes I have checked
> that the voltages at the pins are stable).
>

how about the metal stickers supplied with the floppy disks ?
it's a thin metal layer with adequate glue to keep it anywhere

bye
charley

1996\12\06@013404 by Werner Terreblanche

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Subject: Re: PIC light sensitivity

>> D. L. Davis wrote:
>>
>> I built an altimeter with a PIC14000 which I have in bench test now.
>>  I am using the 16 bit A/D of the PIC14000 to measure voltage from
>> an air pressure sensor.  It is used to control an autopilot, and the
>> PIC runs the autopilot servo too.


> Keep in mind that if you are using a silicon based pressure sensor, it
> will also be sensitive to light. I have seen several altimiters built
> for model rockets which had to use light sheilding on the pressure
> sensor.

Amen to that!  I make audio variometers based around the MPX5100A
pressure sensor and that sensor is *very* sensitive to light.  When I first
noticed the pheonomena, I was quite surprized by it, and immeadiately
consulted the databook.   The was no mention of this in the databook.
But this is the first time that I heard someone else also mention
this, and I'm quite relieved to know that its not just my
imagination.   :)

Rgds
Werner
--
Werner Terreblanche   Tel +27 21 7102251   Fax +27 21 721278
.....wterrebKILLspamspam@spam@plessey.co.za (work)  OR  wernerspamKILLspamaztec.co.za  (home)
Plessey SA, PO Box 30451, Tokai 7966, Cape Town, South Africa

1996\12\06@104421 by myke predko

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Sorry, I just sent this note to just Paul...

>D. L. Davis wrote:
[snip]
>>
>> Everything works great with one big exception.... the PIC is extraordinarily
>> sensitive to light!
[snip]
>
>This doesn't surprise me at all.  Silicon is photoconductive, regardless
>of whether you are talking about junctions or MOS capacitors (i.e.,
>gates).  The degree to which a given function is affected by light would
>depend on several factors:
[snip]
>1. Metallization: the Al layer is opaque.
>2. Impedance: lower impedance circuits are less affected by leakage
>3. Location: circuits nearer the center of the die have a wider field of
>view
>
>Whether the A/D is the charge balancing type, current sources, or
>resistor ladders, it's bound to have some relatively high impedance
>sections.
[snip]
>Paul Mathews, consulting engineer
>AEngineering Co.
>.....optoengKILLspamspam.....whidbey.com
>non-contact sensing and optoelectronics specialists

Does this mean that the PIC (I presume 16C7x) could be used as a light
meter?  ie put on a constant voltage (ie from a voltage divider), leave the
window open and output what the ambient light is?

What frequencies would this be best for?  I presume you would want to put in
a ultra-violet filter (any ideas of what to use)?

myke

Today, the commercial sector is advancing computer and communication
technology at a breakneck pace.  In 1992, optical fiber was being installed
within the continental U.S. at rates approaching the speed of sound (if
computed as total miles of fiber divided by the number of seconds in the year).

Aviation Week and Space Technology, October 28, 1996

1996\12\06@123916 by optoeng

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myke predko wrote:
>
> Sorry, I just sent this note to just Paul...
>
> >D. L. Davis wrote:
> [snip]
> >>
> >> Everything works great with one big exception.... the PIC is
extraordinarily
{Quote hidden}

I try to avoid designing things that depend on unspecified parameters,
so I hesitate to get into this.  Since many of the PICLISTers seem to be
in love with integrated (i.e., expensive but easy to apply) solutions, I
would point them toward the Burr Brown and TI light metering chips with
pulse outputs.  Or, if you're really designing low cost products, a
reverse-biased photodiode charging a capacitor works well.  You can also
use an LED as a photodetector, so a clever designer could use an
indicator (turn it off, too briefly for human eye response, and measure
reverse current).


>
> What frequencies would this be best for?

Depending on junction depths, surface coatings, and so on, most Si
structures have peak responsivity at 950nm, but sensitivity extends from
250nm or so to 1100nm.

I presume you would want to put in
> a ultra-violet filter (any ideas of what to use)?
>

Ordinary window glass blocks most UV. (Which is why your houseplants get
tall and spindly even next to a window: greenhouses use special glass.)

--

Paul Mathews, consulting engineer
AEngineering Co.
optoengspamspam_OUTwhidbey.com
non-contact sensing and optoelectronics specialists

1996\12\06@204525 by D. L. Davis

picon face
Many thanks to all who replied to my light sensitivity problem.  I did
indeed try the metal tape over the PIC14000 JW part window and the results
dramatically improved over the black tape I had been using.  I am now
getting good solid 13 to 14 bit measurements out of the A/D and it is
working great.  I also lowered the impedance of my sensor output circuit and
added a couple of capacitors which helped a lot.  This PIC14000 is looking
to me like a pretty good device after all.            Dewey Davis

1996\12\31@210959 by Eric Smith

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hoss karoly <@spam@timothyKILLspamspamBEKES.HUNGARY.NET> or charley wrote:
> how about the metal stickers supplied with the floppy disks ?
> it's a thin metal layer with adequate glue to keep it anywhere

Not quite anywhere.  For instance, I've never been able to get them to
stick to the floppy disks.

Eric

1996\12\31@213034 by John Payson

picon face
> hoss karoly <KILLspamtimothyKILLspamspamBEKES.HUNGARY.NET> or charley wrote:
> > how about the metal stickers supplied with the floppy disks ?
> > it's a thin metal layer with adequate glue to keep it anywhere
>
> Not quite anywhere.  For instance, I've never been able to get them to
> stick to the floppy disks.

Heh--I see I'm not the only one.  Personally, what I used to like for write-
protection was "Scotch" tape until I discovered that on newer drives it didn't
work.

Oh well... I wonder whether anyone produces any materials that are transpar-
ent to UV but opaque to anything else.  I have confirmed that a mini-maglite
(flashlight) will stop a 16C622 if it is aimed from a distance of about
three inches and it will start to affect the comparators shortly before that.
Room light seems to affect the comparators as well, though in this particular
application I was using a high-impedance (about 1meg) input and so the light
may just have changed the input impedance.  Using the comparator as a 0-5v
DAC, the light affected the count by about 1% (about 50mv).  For some app-
lications this may not be too bad; for others, this would be crippling.

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