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Handheld Thermocouple Thermometer

Handheld Thermometer for measuring high temperatures

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Since I'm a pizza lover and I got a wood oven, I built this thermometer for checking internal oven temperature

This is a project for making an Handheld Thermometer using a K-type Thermocouple. You can read from 0°C up to 1023.75°C depending on the used thermocouple (most thermocouple reads from 0 t 400°C). You cannot read negative temperatures.

Microcontroller used is an ATtiny85 on the Digistump Digispark. Thermocouple is connected through a MAX6675 amplifier. Readings are showed on a 0.91" (128x32) OLED display. Whole thing is powered using a 9V battery connected on Vin pin of the Digispark. Digispark has a 5V regulator on board and a 5V output used for powering the amplifier and the oled.

Even if the OLED controller works at 3.3V is safe powering the display at 5V. Those OLEDS have a voltage regulator on board that step down to 3.3V. SDA and SCL lines are connected on display to 3.3V through pull-up resistors: remember that I2C pulls only lines to GND and when an high logic level must be sent, device put the gpio in high impedance so the logic level is given by the pullup resistor.

A thermocouple is made for reading high temperatures in critical ambients, so if you need accurate and precise readings, maybe a thermocouple is not good for you and you should consider using a PT100 instead coupled to a MAX31865 or similar.

schematic.jpg

Schematic

JPEG Image - 690.36 kB - 10/18/2021 at 09:50

Preview
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enclosure_box.stl

Enclosure box to 3D print

Standard Tesselated Geometry - 182.11 kB - 10/13/2021 at 20:09

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enclosure_lid.stl

Enclosure lid to 3D print

Standard Tesselated Geometry - 118.44 kB - 10/13/2021 at 20:09

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enclosure_probe_slot_cover.stl

This is the cover for the slotted hole of the box

Standard Tesselated Geometry - 14.93 kB - 10/13/2021 at 20:09

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thermo.ino

Arduino source code

ino - 3.40 kB - 10/13/2021 at 20:08

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  • 1 × Digispark
  • 1 × 0.91" 128x32 OLED display
  • 1 × MAX6675 Breakout Board
  • 1 × 9V Battery
  • 1 × 9V Battery Clip

View all 9 components

  • 1
    Prepare Arduino IDE for Digistump

    About this step I wrote a full article some time ago on my blog. It's in italian language but it is automatically translated. You can found my article here.
    After prepared the ambient, upload the code on the Digistump.

  • 2
    Print parts

    Print the parts (STL file are provided here). I've used a resolution of 0.1: there will be a lot of time but enclosure will be very precise. After printed the part, insert the 4 M3 inserts in the corner holex of the box

  • 3
    Solder cables

    You must solder cables on the boards pads first than fit in the enclosure. Place boards in their slots, take measure for each cable, cut and sold.
    Those are the connections (or see the schematic in the attachments):

    • 9V Battery : (+) => Digispark Vin, (-) => GND
    • MAX6675 to Digispark: SCK=>D4, CS=>D3, SO=>D1, VCC=>5V, GND=>GND
    • 0.91" OLED to Digispark: SCL=>D2, SDA=>D0, VCC=>5V, GND=>GND

    Don't make cables too long or lid will not close properly. Don't attach the thermocouple now: will be the last part to be connected.

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