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16 Channel Thermocouple Multiplexer for Raspi

A auxiliary board capable of handling 16 thermocouples and passing their temperatures to a Raspberry Pi via SPI

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An SPI board which allows temperature measurement from 16 thermocouples to a Raspberry Pi. Temperature is measured using a Maxim 31856. It will handle different thermocouple types attached and a temperature range from -200 C to + 1700 C (Thermocouple Dependant) Example Python software allows the direct display to screen and periodic logging to file.

This project is released under the Creative Commons Attribution licence CC-BY. This license lets others distribute, remix, tweak, and build upon our work, even commercially, as long as they credit us for the original creation.

Measuring Temperature

Thermocouples are thin wires of two different types. The most common are Type T which can measure a range of temperature between -270 to 370C. The unit can handle other thermocouple types for different temperature ranges. Thermocouples are connected together at one end. This end is generally referred to as the Hot Junction. The other end connects to your measurement device and is known as the cold junction. A voltage proportional to the difference between the Cold Junction and the Hot Junction temperature is generated by this temperature gradient. This voltage can be corrected and read as a temperature.

This project allows up to 16 thermocouples to be connected and their temperature read in turn. The normalisation and compensation for the ambient temperature are automatically applied and the end temperatures displayed or logged to a comma separated list in a file. Temperatures are displayed and logged in Celsius. Conversions to Fahrenheit can be done by the end user if required.

PiPlay Temperature Circuit Board

The designed circuit board consists of two Analog Devices multiplexer chips for switching between the attached thermocouples and a Maxim Precision Thermocouple to Digital converter. It also has the termination screw terminals for the attachment of the thermocouples. The board also can detect an open circuit thermocouple and flag an error to the operator.

Raspberry Pi

The project is based on a Raspberry Pi which drives the Serial Peripheral Interface (SPI) to set the Thermocouple being read and to read the temperatures. Any device capable of SPI transactions could be used if the end user prefers

Software

Software is written for the Raspberry Pi in Python which can read and display temperatures to the screen. It can also periodically log to a file in comma separated variable form. That file can then be imported to a spreadsheet and graphed if wanted.

Software is available for download in the Files area of this project. Software is in a Tarball format and a discussion on setting up your environment will be available in the Build Instructions area soon.

Screen Output

When running the screen is updated with the readings from all active thermocouples. The names provided for each channel in the .INI file will also be shown in brackets on the screen. This screen update occurs periodically as set in the variable display_interval in the .INI file.

e.g.



In this example one of the thermocouples is showing a Fault condition.  Channel 1 with the name of Tc 1. This occurs if the thermocouple becomes open circuit or disconnected.

The outputted header shows the time and the number of seconds until the display will update and the number of seconds until the next time temperatures are logged to file. These times are set within the .INI file and a further discussion of that file is shown below.

Configuration (INI) file

Configuring the software to read Thermocouples is done in a text based configuration file. Any text editor is able to edit this file. There are a number of sections which are described below.

Default

These are the global level configuration parameters.

[Default]
# Aquisition and Logging intervals in seconds
display_interval = 300
logging_interval = 600

# Logging On/Off
logoutput = On

# Maximum Log file size in Megabytes When the maximum size is reached a new file will be created and the previous file renamed with a date/time suffix.
logfile_maximum = 1

Much of these settings are self evident. display_interval and logging_interval control how long it is between displaying to the screen and logging to a file respectively. These times are in seconds.  An acquisition cycle takes approximately 10 seconds so you should not set these times more frequently. Also frequent logging can lead to self heating of the thermocouple chip and thus introduce error.

The parameter logoutput will switch on or off the logging to file. The parameter logfile_maximum will...

Read more »

Uncertainty.pdf

This is an analysis of the measurement uncertainty of the device submitted by wisedesigns.

Adobe Portable Document Format - 2.11 MB - 06/07/2020 at 03:36

Preview
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gzip - 4.87 kB - 03/29/2020 at 08:22

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Gerbers TC ver1.3.zip

Gerber files for PCB fabrication

Zip Archive - 90.17 kB - 08/23/2019 at 07:32

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PiPlayV1_3.kicad_pcb

KiCad PCB file ver 1.3

kicad_pcb - 288.32 kB - 08/23/2019 at 07:30

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x-kicad-schematic - 18.93 kB - 08/20/2019 at 07:30

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View all 8 files

  • 2 × U1,U2 ADG1607 Analogue Multiplexer
  • 1 × U3 MAX31856 Thermocouple A/D-SPI interface
  • 8 × C1,C2,C3,C4, C5,C6,C7,C8 100nF Ceramic 1206
  • 1 × C9 10uF Tantalum B Case
  • 4 × P1,P2,P3,P4 8 way 0.1" Pitch screw terminals

View all 6 components

  • 0.4 Version Release.

    piplay03/29/2020 at 08:29 0 comments

    A reported bug that prevented the software starling sometimes and getting caught in the Starting 1 loop has been coded around. Anybody who still sees this bug can you report it please.

  • Test of PCB version 1.3

    piplay12/09/2019 at 04:00 0 comments

    I have tested the V1.3 PCB and it works. My previous observation regarding the +/- symbols was in error and they are in fact the right way around. My next task is to design a enclosure to 3D print. At the moment I have the board inside the cardboard box that my Pi came in. To optimise performance you should minimise temperature gradients on the board. The best way to do this is to enclose it, even in a cardboard box!

  • Bug fix for windows smaller than printed area.

    piplay11/19/2019 at 09:31 0 comments

    A minor bug was fixed that caused the application to close if the hosting terminal window was smaller than the displayed text. This has been resolved by specifying a minimum display window.  Although this prevents the application from closing you may need to manually resize the window after the application starts to see all printed text.

  • Minor Update to Python code

    piplay11/11/2019 at 10:32 0 comments

    A small start up bug fixed and new minor version uploaded.

  • Update PCB to Version 1.3 (part 2)

    piplay08/30/2019 at 07:23 0 comments

    The Ver.1.3 PCBs arrived today. I sent the Gerber files on Monday night and the PCBs were on my doorstep Friday afternoon. The price was AU$2.00 for the boards and AU$27.00 for DHL delivery. 

    I will attempt to get one assembled and tested over the week end.

    I just noticed looking at them that I have the +- symbols on the overlay around the wrong way..........Bugger!

  • Update PCB to Version 1.3

    piplay08/23/2019 at 07:41 0 comments

    Posted version 1.3 PCB file and compressed Gerber files. The PCB has been modified but is untested. It "Should" work but it will b e a couple of weeks before I get around to testing it. If you are not feeling brave, the version 1.2 PCB is still available. It works but has 2 unnecessary resistors that have been removed in 1.3. You can fit any value from 0 to a few hundred ohms and it will work.

  • Update Circuit diagram to Ver. 1.2

    piplay08/20/2019 at 07:43 0 comments

    Updated circuit diagram to Ver 1.2

    The circuit has been revised to reflect the removal of the two series resistors. This is as a result of the observation by Hwansoo Suh questioning the function of these components. They are there to protect the input of the converter chip from excess common mode voltage. As the multiplexer will be destroyed by anything much over +3/-0.6 volts the resistor seems a bit pointless. 

    The PCB is yet to be updated. At this stage if you use the Ver 1.1 PCB any value between 0 Ohm to 1K will work. I will publish an updated version as time permits.

  • Hardware Assembly Instructions

    piplay06/10/2019 at 11:52 0 comments

    Video of Maxim Thermocouple chip added.

  • Software Configuration Uploaded

    piplay05/02/2019 at 11:50 0 comments

    Software configuration instructions have been uploaded.

  • Minor Software Update

    piplay05/01/2019 at 08:18 0 comments

    A minor software update was uploaded to fix a problem with display when temperatures drop from 3 digit to two digit. Version number was not updated only date suffix.

View all 13 project logs

  • 1
    Hardware - Attaching the Maxim chip

    The video demonstrates the start of some experiments in improving the performance of the device by improving the thermal coupling of the MAX31856 to the PCB.This will be tested in comparison to an earlier board, eventually.

  • 2
    Software - Setup your Raspberry Pi to run the Thermocouple read program.

    Make sure the SPI subsystem is Enabled

    Run the Raspberry Pi configuration tool

    sudo raspi-config

    When this starts go to the Interfacing Options, then to SPI section. You will be prompted to enable or disable the interface.

  • 3
    Software - Install the needed package libraries.

    Using the Python Package Installer (pip) install the packages required for the project.

    sudo pip install psutil
    sudo pip install configparser
    sudo pip install spidev
    sudo pip install spi

View all 4 instructions

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Discussions

light0494 wrote 06/19/2020 at 18:16 point

Hi, I must tell you, that this is amazing, I was watching a similar product, but, for 4 thermocouples, a 16 channel board like this is very great, I would like to know if you are going to sell this board as a product in a future, or  is just a project for hobby, and, do your software have or will have, the adjust/calibration option for each channel?, I'm from mexico (sorry for my english), I have the idea to mount multipoint system based on something like this project with a touch display, develop an app with an GUI, and some kind of cloud implementation. here is the other board i was watching https://www.mccdaq.com/PDFs/specs/DS-MCC-134.pdf they have a python library in github, anyway, congratulations for this project, it's very great.

  Are you sure? yes | no

piplay wrote 06/20/2020 at 10:03 point

Thanks for your comments, we weren't planning on offering this as a kit or doing much more work on the current software development.  We are not a commercial operation just hobbyists.  The board has shown that it is accurate within +/-  0.5 degrees in testing done  and kindly contributed back by wisedesigns. His report is in the Files section above (uncertainty.pdf)

  Are you sure? yes | no

tonynunn wrote 06/01/2020 at 16:33 point

This is awesome.  Were you able to do any testing to see how much this setup effects the accuracy and precision of the thermocouples?  I know most IC mfgs suggest keeping as short of traces as possible to the connector.  

  Are you sure? yes | no

piplay wrote 06/08/2020 at 04:40 point

A document covering the uncertainty and error correction is now attached. Thank-you to wisedesigns

  Are you sure? yes | no

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