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PICList Thread
'[PIC]: Thermometer CTN'
2000\05\25@154313 by Mark Willis

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Jacky, it's sort of unclear what you're asking here;  Are you a beginner
or do you have experience with PICs and/or Electronics?  I'm guessing
that you want to be able to read a thermistor to an accuracy of 1/10
degree C, over a 0-100 degrees C range, and you're a beginner to PICs at
least?

For this list, I'll pick the 16F84, first off - This's not the Stamp
list <G>

OK, then for 0-100C at 0.1 degree you want a reading of at least 0-1000
from your A/D conversion and some processing, which means you want a 9+
bit A/D reading (assuming the thermistor was linear, which it assuredly
is NOT, so you're going to have to do some extra work, and you may be
able to get by with 9 bits but don't count on it - I'd bet more like
12-14 bit A/D.  Others are more expert here, I'll jsut say that it's
do-able but not a one day project <G>);

How do you plan to handle wiring the thermistor up to your PIC?

How will you get around the self-heating problem with the thermistor?

Will you switch in different precision resistors to scale the
temperature readings, for better accuracy?  (Probably a good plan.
HexFET to ground from each, all common at the thermistor & PIC pin,
other side of the thermistor to it's own precision voltage source
perhaps, or to a cap with a series resistor to at least isolate spikes
off the thermistor readings.)

How will you handle calibration of your precision voltage source
(voltage divider to a PIC pin(s))?  Helps to test your A/D code <G>

Next:  How will you handle calibration of your thermistor to that level
of accuracy?  How much money do you expect to spend, say a couple
hundred dollars or so, to get that accuracy?  Or is the 0.1 degree C
spec a nicety, and if so, what accuracy do you really need?

Finally, you have a temperature, now what do you do with it - Serial
link to a PC?  Log it to an SEEPRom?  You probably know, we list members
don't <G>

There are, fortunately, a lot of good projects to look at to help if
you're a beginner (start at http://www.piclist.com/,
http://www.dontronics.com/piclinks.html has a bunch of good links, so
does http://www.new-elect.com/links.htm.

You might look at the Stamp links if a total beginner;  there may be a
"read thermistor" bit of Stamp code out there, too, I've never bought a
Stamp (they're fairly expensive compared to the 16C622's I first bought
when getting into PICs.)  Bet you'll have to work on it to get 0.1C
accuracy, of course.

I'l suggest you start by searching on piclist.com and the piclist
archives (look on piclist.com), for "sigma delta" (a good way to do A/D,
if a little unclear at first, it grows on you) and for thermistor, those
two search terms should get you more information.

There also are some temperature sensor chips out there, typically only 1
degree C accuracy though.

Also, you will want to be able to program the 'F84 once you have your
code;  There are a good number of different programmers out there.

 Mark

Jacky Joulin wrote:
> Hello,
> I research  soft (and hard ) for read termistor temperature 0 to 100 0C
> Termistor CTN 10K at 250C
> Risolution 1/10 0C
> Hard for PIC 16F84 or Stamp BS2 or ???
>
> tank for your help
>
> spam_OUTWeslayTakeThisOuTspamAol.com

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