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From: rutledge@enuxsa.eas.asu.edu (Shawn T. Rutledge)
Subject: Re: Has Anybody ever built a thermometer?
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Organization: Arizona State University
Date: Tue, 20 Jun 1995 00:09:47 GMT
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: Fekete Vajk (vajk@I_should_put_my_domain_in_etc_NNTP_INEWS_DOMAIN) wrote:
: : Hi!
: : I would like to build a thermometer conneced to my PC.
: : If anyone has already built anything like that, please write me.
: : I am interested in cheap ADC's, and also cannot choose a 
: : sensor. (diode, thermistor...) It has only to work within the -30  +40 
: : Celsius range.

You should be able to hook up a thermister directly to the PC's joystick
port.  It would be the cheapest, also the least accurate way, since joysticks
only have 8-bit resolution and are nothing like calibrated.  About the
best you could hope for would be to check the values you get at various
temperatures, interpolate them for all of the 256 joystick values, and
create a lookup table which turns the joystick value into a temperature.
A joystick of course consists of 2 pots and 2 buttons.  Here is the 
pinout of the connector:  (from Dave Williams' DosRef book-on-disk)

DB15

        1       +5 VDC
        2       button 1
        3       position 0  (X Coordinate)
        4       ground
        5       ground
        6       position 1  (Y Coordinate)
        7       button 2
        8       +5 VDC                  JOYSTICK "A"
----------------------------------------------------------------------
        9       +5 VDC                  JOYSTICK "B"
        10      button 3
        11      position 2  (X Coordinate)
        12      ground
        13      position 3  (Y Coordinate)
        14      button 4
        15      +5 VDC

 The Kraft KC-3 joystick is supplied with two potentiometers. They measure 880k
ohms, probably 1Meg pots. It should be noted that the effective wiper travel is
very limited, say around 45 degrees from stop to stop, and the internal wiring
is arranged so as to leave one end of the pot unconnected. That is to say, the
wiper (middle) post is connected, and one end post is connected as well (I
assume the wires would be called signal and +5v, respectively).

(end of quote)

So, you have 5 volts and ground available jic you need a transistor circuit...
but I doubt it.  Just put a thermistor in place of one pot, and try to
pick a thermistor that goes from about 0 to 1 meg ohms over the temperature
range you want.  If the resistance range of the thermistor is smaller, 
that's when you'll need the transistor circuit.

--
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