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.