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From: ad302@freenet.buffalo.edu (Elizabeth M. Phillips)
Subject: Re: Need help thermometer to RS232?
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Reply-To: ad302@freenet.buffalo.edu (Elizabeth M. Phillips)
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References: <Cx2z9w.Hnu@news.cis.umn.edu> <003302Z02101994@anon.penet.fi>
Date: Fri, 7 Oct 1994 00:01:40 GMT
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In a previous article, roger034@gold.tc.umn.edu (Brynn Rogers) says:

>In article <003302Z02101994@anon.penet.fi>,  <an136163@anon.penet.fi> wrote:
>>Hi,
>>   I am interested in constructing an electronic thermometer which will
>>interface to a PC.  I would like to use the PC to record temperatures over a
>>period of time.  The interface must use an RS232.  Anyone done this before?
>>Are there any magazines that have featured a project such as this?  Any help
>>would be GREATLY appreciated!
>>Best Wishes,
>>Henry.

	This is NOT too difficult depending on how acruate you want and the 
temperatures.

	A good method is too use an RTD device and a 20bit serial A/D convertor.
for example a 100ohm RTD resistance will be 100ohms at 0 C they are acurate BTW
to 100ths of a degree [depending on calibration used, platinum RTD's are best]
thermo couples also work well [good to 10ths of a degree however] but have OTHER
problems [sigh].  Anyhow with some instrumentation OP amps one can get a VERY
precise temperature.  A PIC or some sort of uP can be used to convert the A/D's
output to serial data and then a cheap Rs232 convertor can do the voltage
conversion.

	I would use a Max232 a PIC and a 16 to 20 bit serial A/D combined
with some low power instrumentation OP amps.  The PIC [16c54] can be made
to send/recieve Rs232 data and thus be made programable.  The PIC has a 
RTC time [8 bit divisor] and some sort of prescalor.  By picking the right
clock rate one could have a fairly precise sample rate adjustment.  

	1200buad 8bits 1 stop no parity is the defacto and you can use the
RTS line for handshaking [or waking up :) ] with the PIC.  The cost might
be ... $35 if you do the work.
-- 
