This device is useful for an electric motor that drills holes in printed circuit boards.As long as there is no load on the drill, the rotor will rotate slowly.When the load appears, the rotor will begin to increase speed.
This is due to a voltage drop on the resistor R4. Apply a load to the drill and you will see that there is a voltage drop.This voltage completely opens the transistor S8550, and the drill will start to rotate rapidly
