Automatic switching between 33 and 45 RPM requires a more complex power supply and motor, and when not done correctly can be detrimental to the sound quality of your vinyl.
Rather than complicate the motor arrangement, Pro-Ject turntables with a 16V AC power supply can use the Speed Box S2, which connects directly to the power supply input on the turntable and allows automatic switching. Alternatively, the speed can be manually changed by moving the belt from one notch on the turntable pulley to the other.