The speed of the motor can be varied precise angular positions within the encoder wheel
Encoder wheel consists of Infrared LED and photo transistor is called interrupter circuit
If you apply voltage to DC motor it rotates. Each slit causes the interrupter circuit to generate a pulse by counting the pulse that is displayed in the display
As you increase the DC voltage the speed of the motor increases.
+5V /500mA
+12V /500mA