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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 and 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...

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TCReadV0_3_20190504.tar.gz

Python Code to run board.

gzip - 12.52 kB - 05/04/2019 at 13:16

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BOM.txt

Bill of Materials

plain - 385.00 bytes - 04/25/2019 at 11:51

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PiPlay.kicad_pcb

KiCad PCB file

kicad_pcb - 290.86 kB - 04/23/2019 at 10:27

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PiPlay.sch

KiCad circuit diagram file

x-kicad-schematic - 18.93 kB - 04/23/2019 at 10:26

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Circuit.pdf

Circuit Diagram of 16 Channel TC Mux.

Adobe Portable Document Format - 80.64 kB - 04/23/2019 at 10:23

Preview
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  • 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
  • 2 × R1,R2 470 Ohm 1206

View all 7 components

View all 6 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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