With the S1 switch open, the motor receives a voltage of about 2 V less than that supplied by B1 runs in a smooth regime. To increase its power for an instant, just press S1. 2 to 6 1N4002 diodes can be used for motor currents up to 1 A. The circuit is from the book Robotics, Mechatronics and Artificial Intelligence by Newton C. Braga.
