I2C devices communicate with 2 signals, called SDA and SCL. Normally a 4.7K pullup resistor is connected between each signal and power (+3.3V on Teensy 3.0, +5V on Teensy 2.0). On Teensy 2.0, 4.0, and 4.1, the weak internal pullup resistors may be sufficient for short wires to a si...