An automatic driver for an electric motor. It's needed for convenient drilling of holes in a circuit board. When the drill bit touches the board, the RPM is minimal. This is necessary for convenient positioning of the drill bit. Then, apply light pressure to the drill bit, and it automatically increases its RPM. This means the drill bit's RPM is regulated based on the load. The trimmer potentiometer sets the initial motor RPM. The S8550 transistor is open. When the drill bit is loaded, the voltage across shunt resistor R4 drops, causing the S8550 transistor to close, sending a large current to the base of the BDX53C, causing the drill bit to increase its RPM.
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