You have a motor or dial and want to detect rotation using a rotary encoder.
Use a rotary encoder (also called a quadrature encoder) connected to one of the Bone's eQEP ports, as shown below.
We are using a quadrature rotary encoder, which has two switches inside that open and close in such a manner that you can tell which way the shaft is turning. In this particular encoder, the two switches have a common lead, which is wired to ground. It also has a pushbutton switch wired to the other side of the device, which we aren't using.
Wire the encoder to P8_11 and P8_12, as shown.
BeagleBone Black has built-in hardware for reading up to three encoders.
Here, we use the eQEP2 encoder.
Try rotating the encoder clockwise and counter-clockwise. The values you get for speed and position will depend on which way you are turning the device and how quickly. You will need to press ^C (Ctrl-C) to end the program.