Re: read temperature from i/o port

[dalloff@freenet.columbus.oh.us (Dave Althoff)]

a.jensen@ix.netcom.com wrote:
: For a science project at school, I need to monitor the temperature of
: an object.  I found a program in a science book that will plot a graph
: using a thermisor connected to the i/o port of an apple2.  The book
: doesn't say what a thermistor is, where to find one or how to hook it
: up. I would really apricate any help or advice on what I need or how
: and what pins to connect.
: 	Thanks

A thermistor is a reisitor which has a variable resistance related to
ambient temperature.  I've seen them at Radio Shack in the parts cards,
and those indicate what the resistance is at a particular temperature.

The paddle ports on an Apple ][ are 150k ohm (or was it 150 ohm...I don't
recall off-hand).  So that is the range of resistances you need to work
with.  That may not give you the desired temperature range with the
thermistor; you may need to add correction caps or resistors to make sure
that most of the temperatures you are sensing will fall within the 0-150
ohm range.  Then you'll need to calibrate the program.  Reading the port
is fairly easy from Applesoft, but it will return a value from 0->255.  I
suggest hooking a potentiometer across the paddle port with an ohmmeter so
that you can figure out what resistance is equal to what port reading. 
Then use that with the known resistance of the thermistor for calibration.

That's a start...I don't have my paddle port pinouts handy, or I'd include
those, too.

--Dave Althoff, Jr.
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