2 Thermocouple (MAX6675) on one board

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theKidd
 
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2 Thermocouple (MAX6675) on one board

Post by theKidd »

Can you use 2 MAX6675 thermocouples with one Arduino Uno board? I'm curious if the code library for the thermocouple will support 2 MAX6675 sensors. Thanks.

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adafruit_support_bill
 
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Re: 2 Thermocouple (MAX6675) on one board

Post by adafruit_support_bill »

Yes, the library lets you specify which pins each one is connected to.

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jchristensen
 
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Re: 2 Thermocouple (MAX6675) on one board

Post by jchristensen »

Since the MAX6675 is an SPI device, you can connect any number together on the bus, just wire all the SCK pins together and all the SO pins together and use a separate CS line to each. Just be sure to only select one at a time. I have two wired like this and they work fine. I wrote a very simple library that handles multiple thermocouples and works in conjunction with the standard SPI library, if you'd like to try it let me know.

arroth999
 
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Joined: Sat Nov 26, 2011 3:38 pm

Re: 2 Thermocouple (MAX6675) on one board

Post by arroth999 »

I am in the process of setting up a 9-thermocouple lab system, and I am in need of help! I purchased an Arduino Uno and 9 thin-wire (E-type) thermocouples, as well as 9 free samples of the MAX31855 ADCs. JChristensen, will your advice regarding attaching 2 thermocouples be valid for more? I realize I likely won't be able to read 9 on a single Arduino, but I want to attach the maximum number of thermocouples. Any help would be much appreciated!

Austin

Cadius
 
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Joined: Tue Nov 29, 2011 1:25 pm

Re: 2 Thermocouple (MAX6675) on one board

Post by Cadius »

I've just completed a build using Arduino Mega, Ethernet Shield, and, so far, two Max 6675 K-class thermocouples. I connected the Gnd pins to one of the unnumbered GND pins below pins 52/53 and the VCC pins to one of the two unnumbered 5v pins above 22/23. I then ran the Do, Cs, and Clk to 24, 26, 28 and 26, 27, 29. This works beautifully, and issues the results as a web page formatted:
92-A2-D-A0B-00-00.01: 22.75°c ±2.20 / 72.95°f
92-A2-DA-0B-00-00.02: 23.50°c ±2.20 / 74.30°f
(That's the Ethernet MAC address with a number appended, so I can uniquely identify which MAX I'm reading.)

As I understand it, the Ethernet shield eats pin 4, 10, and 50 - 52 on the Mega, which still leaves enough digital pins to run up to 12 devices. With JChristensen's post about sharing the Clk and Do pins, you should be able to run quite a few!

The margin-of-error values I calculate that using:

Code: Select all

float marginError(float c) {
  //calculate margin/error. 
  //whichever is greater of:
  //if >0 this is 2.2c or 0.75%
  //if <0 This is 2.2c or 2% 
  //per NIST monograph 176: http://www.adafruit.com/datasheets/kthermotable.pdf
  if (c == 0) {
    return 1.1;
  } else if (c < 0) {
    if (abs(c * .02) > 2.2) { 
      return abs(c*.02);
    } else {
      return 2.2;
    }
  } else { // > 0
      if ((c * .0075) > 2.2) {
        return c * .0075;
      } else {
        return 2.2;
      }
    }  
  }

marceln12
 
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Re: 2 Thermocouple (MAX6675) on one board

Post by marceln12 »

hello i want to use also 2 max6675 on a arduino uno , and JChristensen you have written a little lib for that , may i use it from you and where i can find it , or want you to mail it to me , thanks for the helps .

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teo_91bws
 
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Joined: Wed Nov 23, 2016 1:04 pm

Re: 2 Thermocouple (MAX6675) on one board

Post by teo_91bws »

jchristensen wrote:Since the MAX6675 is an SPI device, you can connect any number together on the bus, just wire all the SCK pins together and all the SO pins together and use a separate CS line to each. Just be sure to only select one at a time. I have two wired like this and they work fine. I wrote a very simple library that handles multiple thermocouples and works in conjunction with the standard SPI library, if you'd like to try it let me know.

Hello jchristensen,
I am trying to use two max6675 and I've seen you have already written a working code. Could I have your code please?
Thank you in advance!

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pyr0ball
 
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Joined: Wed Nov 23, 2016 10:13 pm

Re: 2 Thermocouple (MAX6675) on one board

Post by pyr0ball »

Hello jchristensen,

I'm also trying to use a setup with two MAX6675's on a single board simultaneously, and would greatly appreciate access to the library you built.

Thanks in advance!

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