Matthew wrote...

>I am (very) new to the world of PIC's but not to the general world of
>electronics.  My question is: how many K-Type Thermocouples can I connect to
>a PIC 16F84A?

That's like asking, "How many freight cars can I pull behind my
ten-speed bicycle?"

>I am wanting to use the PIC to display the readouts of various temperature
>probes (all K Type), on one single 20x4 LCD screen.  Is this possible with
>this chip or do I need to move up to one with an ADC incorporated?

The PIC16F84A has no innate ability whatsoever to deal with thermocouple
signals, as it does not have any on-chip A/D converter.  You must either
choose a PIC that has an A/D converter of the appropriate resolution
(they come in 8-bit, 10-bit, and 12-bit flavors), or use an external A/D
converter along with the PIC.

As I hinted in my bicycle analogy, the PIC16F84A is also rather
underpowered for the job: it has only 1K of code memory, which is going
to pose quite a challenge to you if you are very new to PICs--especially
if you need to perform linearization (i.e., if you need accuracy better
than about 2 degrees C).  While an experienced and clever PIC programmer
might be able to shoehorn everything into 1K of code space, you, as a
beginner, would be better off choosing something that won't make your
introduction to PICs a trial by fire.  The PIC16F877, for instance, has
8 channels of 10-bit A/D conversion on-chip, as well as 8K of code
space.

>I am aware that I will need one AD595 for each K-Type thermocouple.

Another way to deal with multiple thermocouples, perhaps less expensive,
would be to use a multiplexer ahead of a single AD595.

>Also, how can I extend the leads of the thermocouples without creating
>errors in the voltage created by the difference of the original 2 metals?

This question is answered quite nicely in Analog Devices' application
note AN369, "Thermocouple Signal Conditioning Using the AD594/AD595"; it
also gives a good example of using a multiplexer with the AD595, along
with what's called an "isothermal block" to cancel out the errors that
would otherwise result from thermocouple lead extension.

Another good resource is Linear Technology, Inc. application note AN28,
"Thermocouple Measurement", which provides a good introduction to the
basics of dealing with thermocouples.

National Instruments, Inc. has still more thermocouple measurement
information on their website; there are some very good application notes
and tutorials there.

Hope this helps a bit...

Dave

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