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Raspberry Pi Thermocouple pHat

mikelawrence
mikelawrence
pushedAt 6 months ago

mikelawrence/RPi-pHat-Thermocouple

Raspberry Pi Thermocouple pHat

The Raspberry Pi Zero sized Hats are now officially called Micro-Hats or uHats but for consistency sake, this board's name will remain Raspberry Pi Thermocouple pHat.

This is a Raspberry Pi Micro-Hat PCB that supports:

  • Three MAX31850 1-Wire Thermocouple Converters for remote temperature sensing
  • DS18S20 1-Wire Thermometer for local Hat temperature
  • Magnetic Buzzer/Alert

I keep an upright freezer in my garage and on two occasions I have had the freezer die without getting noticed for several days. This project is an attempt to solve my lack of constant oversight. MQTT is used to communicate with Home Assistant home automation software. The MQTT client/server model is very effective in this situation. Home Assistant will monitor the temperature and provide me with alerts so I can do something about the problem before all the food spoils.

Status

  • Rev 2.0 PCB has been ordered from OSH Park and has been fully tested.
    • Removed one of the four MAX31850K's.
    • Increased thermocouple input filtering by adding ferrite beads and a larger capacitor.
    • Added DC Power connector and switching power supply that powers both the Raspberry Pi and the Hat.
    • The occasional noisy reading from the MAX31850K's are reduced in this version but not gone.
    • You can order parts from Mouser using this shared BOM.
    • You can order the PCB from OSH Park using this link.
  • Rev 1.2 PCB has been ordered from OSH Park and tested.
    • Discovered the MAX31850K's get a noisy reading about 0.1% of the time. This noise is typically within 5 C but sometimes is greater than 30 C. Adding capacitors did little to help the Rev 1.1 design so most of the 3.3V power was switched to a 3.3V LDO regulator in an attempt to reduce the noise on the 3.3V power seen by the MAX31850K's.
    • You can order parts from Mouser using this shared BOM.
    • You can order the PCB from OSH Park using this link.
  • Rev 1.1 PCB was never built.
    • Added a pulldown on the alert signal to prevent the Alert buzzer from sounding on power on.
    • Added an on board DS18S20.
  • Rev 1.0 PCB has been ordered from OSH Park and tested.
    • Discovered the linux kernel doesn't seem to support MAX31850K devices without a DS18S20 present. The kernel detects the MAX31850K devices but does not create a w1_slave file to read the temperature. This is most likely a bug in the Linux W1 driver.

Board Preview

Kicad Notes

  • This PCB design uses my custom libraries available here Mike's KiCad Libraries.
  • This PCB design is based on RPi_Zero_pHat_Template.
  • This PCB was designed with KiCad 5.1.2.
  • For Bill of Materials generation I use my version of KiBoM forked from SchrodingersGat.
  • The LMZ21700 Simple Switcher, AT24CS32, and MAX31850K parts have an exposed pad on the bottom which requires either a reflow oven or hot air to solder properly.

Design

Input Power

This Hat will safely power the Raspberry Pi and this board up to 650mA. Keep in mind that 650mA is not enough for the minimum specified backpower of 1.3A. It is however enough to power (with room to spare) a Raspberry Pi Zero W with nothing else connected. The Barrel Jack will accept 6-11VAC or 6-17VDC. This input voltage is rectified and filtered and applied to a 5VDC Simpler Switcher module from Texas Instruments (LMZ21700). The output of this switcher is applied to the Raspberry Pi 5V through an ideal diode circuit which will prevent any problems when both the Hat and Raspberry Pi are powered simultaneously.

MAX31850K Thermocouple-to-Digital Converter

Although the MAX38150K datasheet typical application circuit doesn't show the use of ferrite beads many designs seem to include them. That in combination with the larger than normal capacitor across the input will hopefully improve sampling errors even further.

Raspberry Pi Setup

This setup makes several assumptions. First you are using Raspbian Buster. This software and instructions most likely work on other versions of Raspbian but they have not been tested. Second Python3 is the target programming environment. It is also assumed that you are using the standard pi user. Otherwise you will have to edit the commands by replacing /home/pi with your user's home directory. Install everything needed by executing the following commands.

sudo apt update
sudo apt -y install git python3-rpi.gpio python3-w1thermsensor python3-paho-mqtt

Get the repository from Github

Clone this repository from Github with the following commands.

cd /home/pi
git clone https://github.com/mikelawrence/RPi-pHat-Thermocouple

Configure ID EEPROM

Raspberry Pi Hats require an ID EEPROM with data that uniquely identifies every hat ever made. Build the EEPROM tools, and make the eeprom_settings.eep file.

cd /home/pi/RPi-pHat-Thermocouple/eeprom/
make all
./eepmake eeprom_settings.txt eeprom_settings.eep

The next command writes the freshly generated and unique eeprom_settings.eep file to the EEPROM but you must push and hold the write switch on the hat before executing this command. By default the EEPROM on the hat is write protected. Pushing the write switch allows writes to occur while the switch is pushed.

chmod 755 eepflash.sh
sudo ./eepflash.sh -w -f=eeprom_settings.eep -t=24c32

You will see the following if writing to the EEPROM was successful.

This will attempt to talk to an EEPROM at i2c address 0x50. Make sure there is an EEPROM at this address.
This script comes with ABSOLUTELY no warranty. Continue only if you know what you are doing.
Do you wish to continue? (yes/no): yes
Writing...
0+1 records in
0+1 records out
117 bytes (117 B) copied, 2.31931 s, 0.1 kB/s
Done.

This is what you will see if there is a problem communicating with the EEPROM.

This will attempt to talk to an EEPROM at i2c address 0x50. Make sure there is an EEPROM at this address.
This script comes with ABSOLUTELY no warranty. Continue only if you know what you are doing.
Do you wish to continue? (yes/no): yes
Writing...
dd: error writing ‘/sys/class/i2c-adapter/i2c-3/3-0050/eeprom’: Connection timed out
0+1 records in
0+0 records out
0 bytes (0 B) copied, 0.0539977 s, 0.0 kB/s
Error doing I/O operation.

If you successfully wrote the EEPROM there is nothing else left to do here.

Setup Interfaces

For this Hat you will need to enable the 1-Wire interface. From the command line type sudo raspi-config and follow the prompts to install support in the kernel.

It's time to reboot your Raspberry Pi with sudo reboot.

Test the 1-Wire Temperature Sensors

Python3-w1thermsensor is a nice 1-Wire python library that also supports command line reading of temperatures from 1-Wire devices. You should have already installed this package in the Raspberry Pi Setup section.

Test the single on-board DS18B20 temperature sensor using w1thermsensor all --type DS18S20.

[email protected]:~ $ w1thermsensor all --type DS18S20
Got temperatures of 1 sensors:
  Sensor 1 (00080372f2c4) measured temperature: 28.06 celsius

Now test the three MAX38150Ks temperature sensor by using w1thermsensor all --type MAX31850K.

[email protected]:~ $ w1thermsensor all --type MAX31850K
Got temperatures of 3 sensors:
  Sensor 1 (000000181928) measured temperature: 23.75 celsius
  Sensor 2 (00000018192b) measured temperature: 24.0 celsius
  Sensor 3 (000000181d59) measured temperature: 2047.81 celsius

The 2047.81 celsius reading is what you get when there is no thermocouple connected to the MAX31850K.

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