A DC Motor Control Mechanism is used to control or change the speed in a DC Motor.  In order for a DC Motor Control Mechanism to operate to its full potential, it needs to consist of the following things:

• A DC power source with three leads, V+, V-, and 0V. 
• Three wires which are connected mechanically to V+, V-, and 0V.
• A potentiometer (a type of variable resistor).  The potentiometer has three electrical leads: two terminal leads, and one called a “wiper” lead.  The position of the “wiper” within the potentiometer determines how much resistance the potentiometer has.
• A DC Motor.

How does the DC Motor Control Mechanism operate?

The DC Motor Control Mechanism operates by varying the voltage applied to the leads of the DC Motor.  For example, if two 6V batteries were used as the power supply, V+ would equal +6 VDC, and V- would equal -6 VDC.  If the potentiometer is adjusted in such a way that the “wiper” is closer to the terminal attached to +6V, the rotor will rotate faster in one direction.  As the “wiper” is moved closer to an equal distance between +6 V and -6V, the rotor will slow down, and eventually stop.  As the wiper moves closer to -6V, the rotor will reverse direction, and rotate faster in that direction.