Re: thermistor project in FAQ

["Michael J. Mahon" <mjmahon@aol.com>]

MechCD wrote:
> Does anyone have some other information about hooking a thermistor to an 
> Apple IIe? I'd love to do so, but I don't know where to start with the 
> programming. The hardware end looks incredibly easy for a change. I saw the 
> FAQ question about this and didn't really get any farther.

You are correct, attaching a thermistor to an Apple II is easy.

Since a thermistor is a temperature-dependent resistor, you should
choose a thermistor whose resistance varies in the 1k - 150k ohm
range in the temperature range of interest.  Thermistors come in
two varieties, those that decrease in resistance with increasing
temperature (negative temperature coefficient) and those that
increase in resistance with increasing temperature (positive
terperature coefficient).

Then attach the thermistor in place of a "paddle" potentiometer,
between +5v and the desired paddle input using the pinout/wiring
information you have already accessed.

The best sensitivity will be obtained when no additional resistors
are used in the connection--just attach the thermistor between +5v
and the desired paddle input.

The resistance of termistors is a nonlinear function of temperature,
so if you need to convert the paddle reading (which is a linear
function of resistance) to temperature, you will need to evaluate
a linearizing function.  (This information is also readily available
on the web.)

If you only need to plot relative temperatures and linearity is not
important, then the paddle input can be used without linearization.

-michael

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