This simple speed control for 3 to 15 V motors uses a common power transistor. The circuit was obtained from my book Mechatronics Sourcebook, published in the United States. The transistor can be the BD135 for motors up to 500 mA, TIP31 for motors up to 3 A and TIP42 for motors up to 4 A. In all cases the transistor must be equipped with a heat sink and the control pot must be wire wound type.
