Making a DC motor spin in reverse is simple. You merely need to swap the power leads that are already attached to the motor. This brief instructional article will walk you through the process. (Even if your leads aren't colored red and black, you will still be able to infer what to do.)
Locate the two power leads that are attached to the motor. One is black and the other is red.
Turn on the power to the motor.
Note which way the motor spins.
Turn off power to the motor.
Reverse the power leads to the motor by attaching the black wire where the red wire was and the red wire where the black wire was.
Turn on the power to the motor. The motor will now spin in the opposite direction.
