The Π^3 (Pi-cubed) Thermometer: a Pi-Shaped, Raspberry Pi-Powered Probe for Perfect Pies

Hey everyone! My name is Taiyo Alex Kobayashi, and I am currently a Mechanical Engineering major at San Jose State University. I am also an active member of the American Society of Mechanical Engineers (ASME) at my university.

I am a greedy goblin. After seeing the "connects to mathematical Pi, Raspberry Pi, or Pie in some way," all of my neurons were activated.

For this project, I wanted to tackle the "All Things Pi" contest by hitting every single requirement in one build. I’ve designed the Pi Thermometer: a device that is Pi-shaped (Geometry), powered by a Raspberry Pi Pico (Technology), and designed specifically for the internal temperature of Pies (Culinary).

This thermometer is designed with a unique foldable housing that is compactable during storage and will folds out into mathematical Pi symbol. Also provides you with handle while you use this tool to check the internal temperature of your Pie :))

This is a great project for anyone looking to practice their Fusion 360 skills, 3D printing, MicroPython coding, and MANY MANY Wiring. I’ve provided all the STL files and code so you can build your own version at home. Let’s get started!

To create the housing, I used PETG filament. I specifically chose PETG over standard PLA because of its superior heat resistance and durability, which is essential for a tool intended for kitchen use near a hot oven. All CAD models were designed in Autodesk Fusion 360, focusing on the mechanical tolerances required for the kinetic folding legs.

I used digital calipers throughout the process to ensure the M4 screw holes and the internal mounting brackets for the Raspberry Pi Pico and OLED were precise.

Hardware & Electronics:

1. Microcontroller: Raspberry Pi Pico (RP2040)
2. Temperature Sensor: K-Type Stainless Steel Thermocouple
3. Amplifier Interface: MAX31855 Thermocouple-to-Digital Converter
4. Display: 0.96" I2C OLED Screen (128x64)
5. Power Switch: SPST Round Rocker Switch (For the main ON/OFF toggle)
6. Power Source: 3.7v 500mah lipo battery
7. Rechargeable: TP4056 Type-C USB charging
8. Fasteners: M4 Screws (Used for the main pivot points and securing the faceplate)
9. Wiring: 22 AWG jumper wires or hook-up wire

Tools & Software:

1. Computer with Autodesk Fusion 360 and your preferred Slicer (e.g., Bambu Studio or Creality Slicer).
2. Thonny IDE (For flashing the MicroPython firmware and code).
3. 3D Printer: (Minimum build volume of 180 x 180 x 180 mm).
4. PETG Filament: (I used gray and blue for the pi theme).
5. Digital Calipers: (Essential for verifying M4 fitment).
6. Soldering Iron & Solder: (To connect the Pico, MAX31855, and the rocker switch).

My goal was to create a device that fulfilled the PI concept (Pi-shape, Pi-powered, Pie-purposed) while maintaining functionality.

Idea 1: The Static Leg

Using the right leg of the PI symbol as the thermal probe itself.

1. Con: This design was incompatible with standard K-type thermocouples, which are rigid. Trying to integrate the sensor into a 3D-printed leg would have likely damaged the internal wiring during use.

Idea 2: The Slicable Slide

A sliding mechanism where the probe extends out from the side.

1. Con: While design is "compatible," this significantly increased the mechanical complexity of the 3D print and wasted valuable internal space needed for the Raspberry Pi Pico and the battery.

Idea 3: The Rotating Joint

A rotating, foldable design using M4 pivot joints.

1. Pro: Most "visually appealing" design while keeping the mechanism simple and durable. It also allowed for a "collapsed" mode for storage and an "extended" mode.

Idea #3 offered the best balance of aesthetics and mechanical reliability.

I used Autodesk Fusion 360 to design the casing and arm/leg for this Thermometer project.

The main goal for this CAD was to create a compact frame that could transform into the pi symbol by rotating the limbs while holding the electronics.

Main Body (Main_Cont.stl) has undergone many adjustments with the complex CAD Module. For the placement of the electronic components, there are many holes/cuts created so they can be slotted into position. Walls/boundaries within the case have been added to define the limits of the limb movement and to avoid any wires getting caught.

Thermal Arm (ThermalArm_Top & ThermalArm_Base) is designed to have the K-type thermocouple to be slotted within the case.

The attachments are in STL file that can be used to print and recreate the project with anykind of modifications.

After the design was complete, I exported the files to my slicer (Bambu Studio, though Ultimaker Cura or PrusaSlicer work perfectly).

1. Supports: I designed the parts to be "3D-print friendly," but would recommend using "Tree Auto" for easy removal after the prints.
2. Filament: I printed the base and faceplate in Transparent PETG and the limbs in Blue PETG to have some color variation. I highly recommend PETG for this project due to its better heat resistance compared to PLA.

1. MAX31855 Thermocouple Amplifier (SPI)
2. Vin: This goes to the 3.3V output pin on the Pico to provide power.
3. GND: This connects to any Ground pin on the Pico to complete the circuit.
4. SCK (Serial Clock): Connect this to GP18.
5. CS (Chip Select): Connect this to GP17.
6. SO (Serial Out): Connect this to GP16.
7. Thermocouple
8. Screwed into the MAX31855 blue terminal block (ensure polarity is correct or your temp will go down when it gets hot!).
9. 0.96" OLED Display (I2C)
10. VCC: Connect this to a 3.3V pin on the Pico.
11. GND: Connect this to a Ground pin on the Pico.
12. SCL (Serial Clock): Connect this to GP1.
13. SDA (Serial Data): Connect this to GP0.
14. TP4056
15. USB-C Charging -> Battery (+) and (-) -> OUT+ to Switch -> Pico VSYS

Software Setup:

1. Install Thonny IDE: Download and install Thonny on your computer.
2. Flash MicroPython: Connect your Raspberry Pi Pico to your computer while holding the BOOTSEL button. In Thonny, go to Configure Interpreter and install the latest MicroPython UF2 firmware.
3. Upload Libraries: You will need two specific library files saved onto the Pico’s root directory for this script to work:
4. ssd1306.py (For the OLED display)
5. max31855.py (For the thermocouple amplifier)
6. For me, I could not find this library so I manually input it. You can do it by opening a new tab, copy-paste the open source library from online and download it to Pico.

I have attached the code that I used for this project below

After making the parts, assemble everything together to produce "The π^3 (Pi-cubed) Thermometer."

Assembly Steps:

1. Component Mounting: I placed the Raspberry Pi Pico and the TP4056 module into their respective mounting bosses in the main body. I used a small amount of adhesive to keep the LiPo battery secure and away from the pivot points.
2. The Faceplate: The OLED and Rocker Switch were press-fitted into the faceplate. I then used M4 screws to secure the faceplate to the main body, being careful not to pinch any of the 22 AWG wires.
3. The Kinetic Pivot: I attached the blue legs using M4 screws and nuts. The right leg houses the K-type thermocouple; I ensured there was enough wire slack inside the joint to allow the leg to rotate from its "storage" position to its "active" position.

A huge error that I made when assembling my circuit was in terms of the length of the cable from my K-type thermocouple. While I thought I would have more space available inside the PI casing than I actually did, I seriously underestimated the amount of braided shield that I would be able to fit. As you can see from the completed pictures, I have quite a bit of extra cable that will not fit inside the casing while retaining kinetic folding capability. The next time around, either a custom-made length of the cable, or the addition of a "spool" section to the main housing would help solve this problem.

It was exciting to put together all those elements in this project: mechanical design, electronics, and some mathematics. The challenge of designing for the All Things Pi contest made me reflect on how to transform the geometric shape into a thermometer.

As a Mechanical Engineering major at San Jose State University, I got to apply my skills and learn how to juggle with two communication protocols (I2C and SPI) in addition to heat-resistant PETG.

This is my first time contributing to Instructables, and it was really helpful. I would like to thank you for taking the time to read up until here and for following my build guide. Should you have any questions regarding Fusion 360 tolerances and MicroPython code

Thank you!