DC Motor Controller Project Kit

There are three ways to vary the speed of DC motors:

Use a gear-box.
Use a series resistor. Measure the current drawn by the motor and then calculate the value of a series resistor needed to drop the voltage applied to the motor. Less voltage means the motor will slow down. The problem with this method is that the current drawn by the motor increases as the load on the motor increases. More current means a larger voltage drop across the series resistor and therefore less voltage to the motor. The motor now tries to draw even more current, resulting in the motor stalling.
By applying the supply voltage to the motor for a variable amount of time. This is the method used in this kit. This is a revised circuit from earlier models.

This document kit will apply the ways no. 3