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PICList Thread
'Cheap Temp Sensors'
1997\03\14@090813 by Ralf Rudolph

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Hello there, PICers!

As a first PIC project, I would like to build a unit that measures 8
Temperature Sensors and reports the results to a serial port (no, that«s not
only for academic reasons: I really need that thing... ;-)

I want to keep the Design as simple as possible. For the A/D Conversion,
after reviewing all the good Ideas from this Mailing List and other Sources,
I finally asked myself if the following design could work (it simply looks
too simple to me - maybe I missed some point?):

I take 8 I/O Pins from any 18pin PIC device and connect them as follows:

RB0 ---[NTC resistor]---*
RB1 ---[NTC resistor]---*
RB2 ---[NTC resistor]---*
RB3 ---[NTC resistor]---*
RB4 ---[NTC resistor]---*
RB5 ---[NTC resistor]---*               cap
RB6 ---[NTC resistor]---*               | |
RB7 ---[NTC resistor]---*---[100 Ohm]---| |---GND
                                       | |

For the Conversion, I program all pins to output High and wait a while to
charge the capacitor (a time loop, lets say 0.5 seconds). Then, I program
all pins except the one I want to measure to be input, and the probant pin
to be output at low level. Then I take the time it takes to discharge the
capacitor through the measured NTC resistor (I can monitor that through all
the other pins that are now inputs - this trick saves me an extra input line).

The resistor in line with the capacitor shall limit the maximum current
while charging the capacitor (to protect the PIC). BTW, how would you
calculate the capacitor? I thought of about 10uF, but that«s just a wild
guess... A sampling time of 1 or 2 seconds is perfectly fine with me. An
accuracy of 0.1 degrees would be perfect, 0.5 degrees would also be sufficient.

My question is now, since I never used PICs before (actually, I would buy a
development kit just for that design): Do you think this could work?

Thanks in advance for all Responses!

PS: Last time when I asked for the "PICPLUS" Development system I forgot to
say Thanks to all those who gave me valuable hints. I am Sorry! Here I go
now, a little late: "Thanks!"


--

| Ralf Rudolph
| Dipl.-Inf. (FH)
|
| Artifex Editorial Services GmbH
| Johanna-Melber-Weg 4
| D-60599 Frankfurt / Germany
|
| Phone: +49-69-962217-24
| Fax: +49-69-610351
| E-Mail: spam_OUTralfTakeThisOuTspamartifex.de
| CompuServe: 101234,53

1997\03\14@225045 by Michael S. Hagberg

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sometime i wonder how much trouble one will ge through to save a buck.
i'd say just use LM34 temperature to voltage cost $1.25 USD and the
a/d and uart build into the pic16c74. i did a project like this that
displays all eight readings and controls relays and even changes setpoints
by reading switch contact inputs. it was lots of fun and done only with
the software simulator DOS version cause it works best.

michael

1997\03\15@083913 by Michael N. Steen

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At 21.43 14-03-1997 -0600, you wrote:
>sometime i wonder how much trouble one will ge through to save a buck.
>i'd say just use LM34 temperature to voltage cost $1.25 USD and the
>a/d and uart build into the pic16c74. i did a project like this that
>displays all eight readings and controls relays and even changes setpoints
>by reading switch contact inputs. it was lots of fun and done only with
>the software simulator DOS version cause it works best.
>
>michael
>
>
I fully agree to the above, and I will add that by using NTC's you would
probably have to calibrate all 8 individually to get the required accuracy,
because they are usually not linear nor accurate to the degree you want.

Michael

1997\03\15@200046 by Karoly Hoss

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Ralf Rudolph wrote:
>
> Hello there, PICers!
>
> As a first PIC project, I would like to build a unit that measures 8
> Temperature Sensors and reports the results to a serial port (no, that4s not
> only for academic reasons: I really need that thing... ;-)
>
> I want to keep the Design as simple as possible. For the A/D Conversion,
> after reviewing all the good Ideas from this Mailing List and other Sources,
> I finally asked myself if the following design could work (it simply looks
> too simple to me - maybe I missed some point?):
>
> I take 8 I/O Pins from any 18pin PIC device and connect them as follows:
>

why don't you use a ds1820 ?
only one 5k resistor and it's done .
and you'll need only one data pin

bye
charley

1997\03\16@154744 by Stein Sem-Jacobsen

picon face
> Ralf Rudolph wrote:
> >
> > Hello there, PICers!
> >
> > As a first PIC project, I would like to build a unit that measures 8
> > Temperature Sensors and reports the results to a serial port (no,
that4s not
> > only for academic reasons: I really need that thing... ;-)
> >
> > I want to keep the Design as simple as possible. For the A/D
Conversion,
> > after reviewing all the good Ideas from this Mailing List and other
Sources,
> > I finally asked myself if the following design could work (it simply
looks
> > too simple to me - maybe I missed some point?):
> >
> > I take 8 I/O Pins from any 18pin PIC device and connect them as
follows:
> >
>
> why don't you use a ds1820 ?
> only one 5k resistor and it's done .
> and you'll need only one data pin

Can you get the ds1820 in a sealed package?

-Stein Sem-Jacobsen

1997\03\16@191110 by Karoly Hoss

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Stein Sem-Jacobsen wrote:
>
> Can you get the ds1820 in a sealed package?
>

what do you mean by "sealed" ?
it's very much like a transistor , or if you use the can-type
it'll look like a double sized clock battery

bye
charley

1997\03\17@185246 by Stein Sem-Jacobsen

picon face
> Stein Sem-Jacobsen wrote:
> >
> > Can you get the ds1820 in a sealed package?
> >
>
> what do you mean by "sealed" ?
> it's very much like a transistor , or if you use the can-type
> it'll look like a double sized clock battery
>
> bye
> charley

I mean like shielded from the outside environment. So I can use it outdoor,
-exposed to rain.

Sem-Jacobsen

1997\03\17@212706 by James Widman

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> > > Can you get the ds1820 in a sealed package?
>
> I mean like shielded from the outside environment. So I can use it outdoor,
> -exposed to rain.

 Try coating it with a marine epoxy, I've done it and it worked fine.
 As long as you don't need fast response times to rapid temperature changes
 all should be well.  I'll be using it to measure water temperature and
 there will only be gradual temperature changes so I'm not worried about
 the additional thermal mass.

 Good Luck

 Jim Widman

1997\03\18@040516 by Keith Dowsett

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At 22:20 17/03/97 +0100, you wrote:
>> Stein Sem-Jacobsen wrote:

>> what do you mean by "sealed" ?

>
>I mean like shielded from the outside environment. So I can use it outdoor,
>-exposed to rain.
>
>Sem-Jacobsen

Why not pot it yourself after testing?

Won't leaving it in the rain generate some spurious results? Whenever the
sensor gets wet it will be cooled by evaporation.

Keith.
==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: .....kdowsettKILLspamspam@spam@rpms.ac.uk
  WWW: http://kd.rpms.ac.uk/index.html

1997\03\18@183958 by Karoly Hoss

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Stein Sem-Jacobsen wrote:
>
> > Stein Sem-Jacobsen wrote:
> > >
> > > Can you get the ds1820 in a sealed package?
> > >
> >
> > what do you mean by "sealed" ?
> > it's very much like a transistor , or if you use the can-type
> > it'll look like a double sized clock battery
> >
> > bye
> > charley
>
> I mean like shielded from the outside environment. So I can use it outdoor,
> -exposed to rain.
>
> Sem-Jacobsen

I had to use it in oil and water , rain , and dust
I put it in a metal can with a simple telephone cable attached and
isolated
it with epoxy . it works fine . if you need quick response I'd use
shrink tube only
anyway if you use it with three wire (vcc gnd data) the signal to noise
ratio is
about 50kohm to 50 ohm so you'll have plenty reliability to waste

bye
charley

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