> -----Original Message-----
> From: T.C. Phelps [SMTP:tcphelps@YAHOO.COM]
> Sent: Monday, March 04, 2002 5:58 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC]: Re: Beginner Q. How many K-type thermocouples can the
> 16F84A handle?
>
> Matthew,
>
> Yep that's definitely possible with the F84A. I'm
> working on a similar project (but with analog
> acceleration inputs instead of thermocouples). So
> here's a suggestion -- get a multi-channel ADC. For my
> current project I'm using Microchip's MCP3202 (if I
> remember the part name correctly), which has two
> separate channels and lets you send 12 bit ADC values
> over an SPI serial connection, which is only four
> wires -- perfect for the 16F84's PORTA (and an LCD can
> go on PORTB -- I'm using a Seiko 1671 16x1, which uses
> up 7 of PORTB's bits). I'm sure you can find 4 channel
> ADCs or more out there that connect over serial lines
> (number of channels=number of thermocouples you can
> connect up). I've worked with K-type thermos before,
> you'll most likely have to connect it up to an op-amp
> based amplifier (you can get instrumentation
> amplifiers on a chip, they're really good for those
> low voltage levels) then send the amplifier output to
> the ADC. As for extending the leads, the voltage
> output of the thermocouples is already small, so to
> save yourself a lot of headaches you're probably best
> off just ordering the length of thermocouple wire you
> need. You can get it in pretty much any length (just
> make sure that the ends have already been welded, and
> you're not getting two rolls of unconnected wire). If
> you solder stuff onto the end to make a short
> thermocouple longer, you're going to throw your
> measurements way off. Hope that helps you out...
>
Don't forget that you have to have some kind of cold junction compensation
if you want even slightly accurate results.

Mike

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