Cheapest and easiest way to slow down a motor

[Al Skierkiewicz]

There are only three parts, the battery on the left, a pot in the center and a transistor on the right in series with the motor. The transistor will need to be mounted to a heatsink because it will get hot. As you move the pot, a varying amount of current will be supplied to the transistor base thereby controlling the current in the collector/motor. With the wiper all the way up at 12 volts, the motor will be at max speed. Start with a 10K pot but you may need a 100k pot depending on the range you need and the gain of the transistor.
As an alternative, you could use the speed control from a 12 or 14 volt drill too or the motor control from a car blower which is usually three resistors and a switch.