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Pax Instruments T400 thermocouple temperature datalogger

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The T400 was featured on the Mailbag and got a thumbs up!  8)

If you're interested in influencing what might be the next device, please join the conversation at https://www.eevblog.com/forum/projects/pax-instruments-multilogger/

In the latest firmware in github gives 770 bytes available without removing any functionality via flags. If flags are used, a total of 9,570 bytes are available.


I realize this thread is old.  Charles Pax has kindly helped me out with finding a project box for my "Smart Roast" and asked that I post some pictures of how I repurposed the T400 box.  (See related thread here: edit:deleted link because of Chrome warning)  This project is/will be open source.  Code is MPASM.  I just have not had time yet to get that documentation cleaned up, but I will add it here.

My project used a PIC16F1789 and MAX31856 TC amplifier.  The rest is pretty standard support stuff.  I went with an Omega TC socket, and it was just a tiny bit too high to fit under the LCD that snaps into the board.  That was solved by making a slot in the board and effectively mounting it upside down on the bottom surface.  See attachment. 

The thing is, of course, a roast thermometer.  I added a clock for total time, a calculation of rate for internal temperature, and a calculation of finish time based on user input of desired final temperature.  With beef roast or pork and an oven temperature well above the target temperature (e.g., 325°F vs. 125°F/160°F) that rise in temperature was fairly linear shortly after internal temperature starts to rise.  I had noticed that linearity for years and used to do the calculation by hand.  I could never convince my children to keep such records, so I made this project for them.  I now like it too.

The case back panel is currently left open for access to the programming pins.  Once that is "done," it will be closed.  The other attachments are for the TC (type K) and battery charging/power (USB).  For the first prototype, I decided not to include on-board charging and have left the battery unattached.  Nevertheless, I have included a picture of the T400 battery compartment, which I really liked.

PCB back shows detail of how the TC socket is attached.
PCB w.LCD shows detail of how the LCD mounts to the PCB.
MAX31856  shows the top attachment of the TC socket.  I used cyanoacrylate adhesive to fixture the socket to the PCB strap on the backside.  It seems quite rugged.
PCB top is the whole board with the LCD flipped forward.
The final photos is just the PIC1789 for those who like to see the MCU.

I extend my sincere public thanks to Charles Pax.  Without the T400 case, getting this gadget to look so neat may not have happened.

Regards, John

Penultimate SmartRoast Prototype (maybe)

I have mentioned my "SmartRoast" project in other threads.  At the suggestion of Charles Pax (see my previous post), I decided to post more detail here.  It's a roast thermometer that calculates a finish time from the user entered target temperature.  It does not predict temperature rise after removal from the oven.  The basic project was working within one Winter.  I then spent the next 4 years working on the graphical display, PCB, and perseverating about when to do my very first reflow soldering (done 2022).  Other projects were also done during that time.  My original PCB design included a BMS for a LiPo battery; however, that was deleted at the last minute as I became worried about heat and keeping the project simple for my first reflow attempt.  It runs on a 5V wall wart with a miniature USB plug. 

Changes anticipated or being considered for the next version: 1) On board BMS with a USB-C connector (definite); 2) Selection of meat type; 3) An initial calculation based on size and weight of the roast;  4) Change the MCU chip; 5) Change the TC amplifier and/or better noise filtering; and 6) Possible but less likely a change in the display.

Attached are the schematic and board design submitted to OshPark, Assembly code, some flowcharts that may help a reader find their way through the spaghetti "logic," and a link to the LCD that was used.

1)  Coding is in Microchip Assembly and MPLab 8.92 tool suite.  It may not appear in text or assemble properly with the current assembler in MPLAB X IDE.
2)  RE: TC amplifier (MAX 31856).  That chip is sensitive to noise on the TC leads and can freeze up.  If that happens, turn device off, wait a few minutes, and restart.  I found that wrapping the TC leads through a ferrite filter/toroid helps.  The toroid is shown in some of the pictures.
3)  Flowcharts were done originally in draw.io which is now diagrams.net .  I continue to use the draw.io version.  The Histogram flowchart has not been updated in quite awhile and should be considered a rough draft.
4)  The LCD is not shown in the BOM.  It was obtained via AliExpress from Laurellcd.  It is described as: 13264 COG ST7567 lcd display graphic module SPI Serial 12pin FSTN gray with bright backlight serial module LG132643-FDW .  Note the FFC comes/came in 2 pitches.  The prototype has the narrower pitch.  The plastic legs snap into oval slots on the PCB
Link: edit link to avoid Chrome warning
User's Manual:
The switches are numbered top to bottom and left to right.  Upper left is SW1; lowest left is SW3.
During POR
SW1 = Run
SW2 = Menu select
SW3 = Power on/off
SW4 = Increase in Done temp
SW5 = Decrease in Done temp
SW6 = Inactive (not implemented)

During Run
SW1 = Clears screen and timer (settings unchanged from POR)
SW2 = Backlight on/off toggle
SW3 = Power on/off
SW4 = Command scroll up
SW5 = Command scroll down
SW6 = Inactive (not implemented)

Regards, John


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