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DC motor direction control
This circuit uses 2 NPN and 2 PNP transistors to form a DC motor direction control. When switch S1 is in the FORWARD position, Q1 is driven on through R1, and Q2 is driven on through R3 by Q1. However, Q3 is cut off through R4, and Q4 is cut off by R5 and R6. Thus the "live" side of the motor is connected through Q2 to the positive supply, and the motor runs in the forward direction. When S1 is in the OFF position, Q1 is cut off through R1, and Q2 is cut off through R2 and R3. Simultaneously Q3 is cut off R4, and Q4 is cut off through R5 and R6. Under this condition, the "live" side of the motor is open-circuited so the motor does not run. Finally, when S1 is in the REVERSE position, Q3 is biased on through R4, and Q4 is driven on through R6 and Q3. However, Q1 is cut off through R1, and Q2 is cut off through R2 and R3. Therefore, the "live" side of the motor is connected through Q4 to the negative power supply, and the negative power supply, and the motor runs in the reverse direction.