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'Optical Sensor'
2009\05\28@231008 by Benjamin Grant

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Hi all,
this is probably something silly i'm overlooking btu I have a custom built
optical sensor. When I measure across it with an oscilloscope, I get a range
from 0 to 5V.  If I then hook it up to a Ni DAQ card, the computer always
reads it as high... measuring in parallel the range is now only 2 V to 5
V... any idea why it's unable to drive to ground now? thanks a lot
Ben

2009\05\28@234939 by Harold Hallikainen

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> Hi all,
> this is probably something silly i'm overlooking btu I have a custom built
> optical sensor. When I measure across it with an oscilloscope, I get a
> range
> from 0 to 5V.  If I then hook it up to a Ni DAQ card, the computer always
> reads it as high... measuring in parallel the range is now only 2 V to 5
> V... any idea why it's unable to drive to ground now? thanks a lot
> Ben

I wonder what the input of the DAQ card looks like. It may have a negative
bias current that causes a positive input voltage when driven by a high
resistance. Does the DAQ read zero if the input is open circuited?
Depending on what you're driving this with, you may be able to add a
pull-down resistor to fix the problem.

Harold


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2009\05\29@000720 by Benjamin Grant

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Thanks Harold,
the default open circuit state is high.  I was thinking a pull down resistor
might be necessary as well. thanks
ben

On Thu, May 28, 2009 at 11:59 PM, Harold Hallikainen <spam_OUTharoldTakeThisOuTspamhallikainen.org
{Quote hidden}

> -


'Optical Sensor'
2009\06\01@171056 by Herbert Graf
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On Thu, 2009-05-28 at 23:10 -0400, Benjamin Grant wrote:
> Hi all,
> this is probably something silly i'm overlooking btu I have a custom built
> optical sensor. When I measure across it with an oscilloscope, I get a range
> from 0 to 5V.  If I then hook it up to a Ni DAQ card, the computer always
> reads it as high... measuring in parallel the range is now only 2 V to 5
> V... any idea why it's unable to drive to ground now? thanks a lot

What's the output impedance of your sensor? We've used NIDAQ cards
before and have never seen anything like that. That said, we always
drive the NIDAQ cards with "normal" drivers. You may have to add an
op-amp in unity gain mode to get it to work properly, it sounds like the
card has a weak pull up that your sensor can't pull down.

TTYL

2009\06\01@212518 by Benjamin Grant

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Thanks Herbert,
it turned out to be an intrinsic problem with the sensor - there was an
indicator LED that was causing the signal to jump to 2 V.  Not exactly clear
why this wasn't evident on the oscilloscope. Regardless, ended up utilizing
a transistor and created a logic gate and all is well. Thanks
Ben
On Mon, Jun 1, 2009 at 4:59 PM, Herbert Graf <.....hkgrafKILLspamspam@spam@gmail.com> wrote:

{Quote hidden}

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