From: Spehro Pefhany
Subject: Re: Thermistor thermometer w/parallel port connection
X-Newsreader: Forte Agent 1.92/32.572
Date: Thu, 09 Jan 2003 16:26:22 GMT
NNTP-Posting-Date: Thu, 09 Jan 2003 11:26:22 EST
On 9 Jan 2003 07:55:51 -0800, the renowned firstname.lastname@example.org
>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!
Suggest you partition the project into an ohmmeter with parallel port
interface and a QBASIC program that produces temperature from
resistance in accordance with the equation for your thermistor.
Use lots of bits unless your temperature range is narrow.
For example, you could measure the time to charge a capacitor to
a fixed voltage through your thermistor and divide it by the time to
charge a reference resistor to the same voltage. The time
measurement could be done with a counter with 3-state outputs,
maybe 12 bits. The division and control logic can be handled by
the computer, as can the fault detection (time-out). This moves
a bit of the ohmmeter into the computer, but that's probably okay.
Presumably 'subsystem' includes software subsystems, but you might
want to ask your instructor.
"it's the network..." "The Journey is the reward"
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