At 10:00 AM 7/22/97 GMT+0200, you wrote:
>> I am looking at designing a simple project which will simply sample the
>> temperature and store the maximum temperature reached as well as the
>> average.
>> What do people recommend for temperature sensors (please also give a place
>> where I can buy them in low quantity. ( < 10))
>> Would you use a thermoresistor hooked to a PIC that had an on-board A/D or
>> would you use a Dallas (or equivalent) digital sensor?
>> I only need approximate (2 degrees or so) and it is to put on top of a
>> roof.  (The rest of the circuitry besides the sensor will be kept inside
>> their specs.)
>>
Snip

Whats wrong with a 25 cent thermistor and a series 1 % resistor to the +5
supply and possibly a shunt resistor to control range.  The curve of the
thermistor can be linearized by a small look up table in memory or by
scaling according to the magnitude of the output of the 16C71 or equiv.
on-board A/D converter.

I have used a 10K 2% thermistor over and over for ranges of -20F to +170F.
Depending on the size of the table, accuracy can be accomplished to about
+/- 3 degrees over the above range.  Naturally resolution is 8 bits.  If
good accuracy is a must then the VDD supply must be reasonably well
regulated and the absolute value relatively close to +5V.  Any VDD can be
used but the table will have to be adjusted to accomodate the different
Vref.  One could use one of the A/D channels to read the VDD and provide
software scaling.

Digikey is a source for Panasonic and other Manuf for thermistors.

Remember that if you use a negative coeficient thermistor and the thermistor
leg is on the VSS side then your reading will be inverted.  (higher the temp
- lower the A/D reading.  If you need tracking in the same direction put the
thermistor in the VDD leg or use a positive temp coeficient device (more
expensive).  Use software to linearize the thermistor reading which
typically is,if I remember correctly, is about .7%/Deg. C.

By the way, if your application is to determine specific limits, you can use
the A/D outputs without scaling.  Simply make the A/D reading the limit for
the Temp limits you wish to establish.

One other point.  Keep the current low through the thermistor so self heat
will not contaminate the reading.  50 mW is usually OK for non precision
applications.


Gerald Lewis  k2mfh@worldnet.att.net