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'Temperature sensing and the 16F876...'
2000\02\01@205230 by Erik Reikes

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I have a desire to do some really rough-scale temperature sensing with the
10 bit AD in my 16f873.  I am looking for something like +/- 1-3 deg. C
over the range 0-100C.  I think I can do a single point cal at the time of
manufacture.

Here's my initial idea : find a stock resistor with a really crappy temp
co. and measure a voltage across it.  At 300PPM/deg. C, that gives me a
delta of 3% over the temp range.  If I did some clever voltage referencing
I think I ought to be able to get at least 100 clicks in AD over that
range, I just can't think of how.  Keep in mind the operating word here is
cheap.  We want to temp sense stuff inside of our cases so that if there
are failures we can go back and look at what the reported temp was in the
box and see what is killing us.

Thermistors seem to be in the $1-$2 range, and I'd like to be in the
$0.10-$0.20 range.

You guys got any ideas?

Are crappy temp co. resistors linear, or at least monotonic at those temp
ranges?

Maybe I'd get a better temp variance with a capacitor and timing an RC
delay....

TIA

-E
Erik Reikes
Software Engineer
Xsilogy, Inc.

spam_OUTereikesTakeThisOuTspamxsilogy.com
ph : (858) 535-5113
fax : (858) 535-5163
cell : (858) 663-1206

2000\02\02@003428 by Dennis Gearon

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a xister makes a cheep temp sensor

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   Sincerely, Dennis Gearon

2000\02\02@032230 by Robert A. LaBudde

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<x-flowed>At 05:51 PM 2/1/00 -0800, you wrote:
>I have a desire to do some really rough-scale temperature sensing with the
>10 bit AD in my 16f873.  I am looking for something like +/- 1-3 deg. C
>over the range 0-100C.  I think I can do a single point cal at the time of
>manufacture.
><snip>
>Thermistors seem to be in the $1-$2 range, and I'd like to be in the
>$0.10-$0.20 range.

1. A silicon transistor diode-wired (base to collector) or a silicon diode.
This item has about a -2.5 mV/C temperature coefficient is acceptably linear.

2. A thermocouple (twist or solder together two different metals, e.g.,
copper and nichrome). Cost is very low, has linear temperature coefficient
of a few tenths of millivolts/C.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: .....ralKILLspamspam@spam@lcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
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</x-flowed>

2000\02\02@142338 by jamesnewton

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>From the PICList FAQ at
http://www.piclist.com

Use the PICs Watchdog Timer as a temperature sensor! Its frequency changes
linearly over temperature, so it's possible to build a thermometer without
external chip! See Microchip App Note AN720. They claim accuracy up to 1
degree Celsius.

---
James Newton jamesnewtonspamKILLspamgeocities.com 1-619-652-0593
http://techref.homepage.com The Technical Details Site.
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{Original Message removed}

2000\02\02@150251 by Erik Reikes

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At 11:22 AM 2/2/00 -0800, James Newton wrote:
>>From the PICList FAQ at
>http://www.piclist.com
>
>Use the PICs Watchdog Timer as a temperature sensor! Its frequency changes
>linearly over temperature, so it's possible to build a thermometer without
>external chip! See Microchip App Note AN720. They claim accuracy up to 1
>degree Celsius.
>
>

Interesting idea, and about as cheap as you can get...Hmmm....

I originally was going to calibrate my WDT for temp in order to get better
accuracy in sleep mode (I'm using the WDT to wakeup).  I canned this after
realizing that skewed sleep times were advantageous, but I think that code
won't go to waste now.  Great idea and thanks for the pointer!

-Erik Reikes
Erik Reikes
Software Engineer
Xsilogy, Inc.

.....ereikesKILLspamspam.....xsilogy.com
ph : (858) 535-5113
fax : (858) 535-5163
cell : (858) 663-1206

2000\02\02@200413 by Donald L Burdette

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>I have a desire to do some really rough-scale
>temperature sensing with the10 bit AD in my
>16f873.  I am looking for something like +/- 1-3 deg. C

>Keep in mind the operating word here is cheap.

IIRC, the way temperature sensing is done on Pentium chips is by
measuring the forward voltage on a PN junction.  This voltage is very
stable, predictable, and motonically temperature dependant (though I
don't know about linearity).

If you use a simple 1N914 and LM324 with five resistors to get gain of 20
and input reference of .5V, you should get a signal that ranges from
about 1 to 4 V over -40 to +125 deg C.  I don't think you have to
calibrate it, at least if your resistors and op-amp are accurate enough.
It's about US$0.25 worth of parts in volume, maybe less.

Of course these numbers are rough, off the top of my head.  I'm sure
someone can tell you where to get better numbers if you decide to go this
way.

Don

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