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From: "R Wilcock"
Subject: Re: Thermistor thermometer w/parallel port connection
Date: Thu, 9 Jan 2003 18:12:37 -0000
X-Newsreader: Microsoft Outlook Express 6.00.2462.0000
Clever way to avoid using an a/d is to make the computer output a PWM signal
and then pass that into a low pass filter - that will give you a dc level
equivalent to the average value of your PWN signal then you just gradually
change the PWM signal so that the DC level changes and you use a comparator
to compare this to the output of a simple divider made with your thermistor
and you know when it changes state that that is the voltage level so you can
calculate the resistance. you can make a PWM easily if you have a free
running counter and a compare value. Say if you have an eight bit free
running counter then you just compare the value to a compare value and make
the output high if it is greater and low if lower.
Sorry not makeing much sense probably. Obviously doing it with a ADC is much
better but i thought i would throw this suggestion to you - its especially
useful if you use microcontrollers which do not have an on-cip ADC but do
have an analog comparator and you need to knock up somthing which can figure
out the value of an analogue signal. The low pass filter obviously has to
filter out everthing above and including the frequency of the PWM signal so
you have to understand how to design a Low Pass Filter.
A better way to do what you want to do (i guess i havent made much sens
above) and much more accurate is to use a thermocouple. You stick a
thermocouple into a nice little thermocouple amplifier chip (which will
measure temp very accurately) and then that into an 8 bit parallel ADC which
is clocked from a clock. Then its really easy just to read off the values.
You also have some nice blocky subsystems (if thats what you mean by that).
Let us know how you got on
"Luke" wrote in message
> Hi all,
> I've read loads of posts about thermometers but none like this:
> I would like to design a thermomter (preferably using a thermistor) to
> connect to PC by parallel port. It's for my A-level electronics
> project, and needs AT LEAST 3 subsystems, so although there are simple
> 1-chip 2-capacitor & 2-resistor solutions, I need to go the whole hog
> and design pretty much from scratch! And yes, I DO want to use QBasic
> and not another language, it's on the syllabus.
> I'm not asking for a complete solution, just some hints from you more
> experienced folk about how would be best to go about it! Oh, and
> links to relevant sites would be great as well!
> Thanks a lot,
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