There are 16 white LEDs arranged as two 7-segment digits, separated with a colon on the QT5 Xplained Pro extension board that can be used to visualize touch on the QTouch slider sensor. All LEDs on the board are driven by the LED matrix driver (IS31FL3728), which is controlled by I2C. For detailed instructions on how to interface the LED matrix driver, refer to the devices datasheet at http://www.issi.com/.