I’ve set up my Raspberry Pi to communicate with my Arduino via I2C. The Raspberry Pi is a 3.3v device and the Arduino is a 5v device. While in general this means that you need to use a level converter when connecting the two devices, you don’t need to use a level converter when connecting the Arduino to the Raspberry Pi via I2C. The design of the I2C bus is such that the only device driving a voltage on the bus is the master (in this case, the Raspberry Pi), via pull-up resistors.