The simplest and cheapest way to control speed of DC motor is by using variable resistor connected in series with motor.
This method is applicable to any low power DC motor like 3V pager motor. Direction of rotation is selected by switch.
If you want to use chip cooling fans in your design, keep in mind that it's impossible to reverse cooling fan simply by switching power polarity since fan assembly incorporates electronic driver which is designed to maintain motor speed and direction of rotation.
|Motor control. Simplest way.|
Yes, it works but from electrical point of view it's a waste of energy.
The more advanced way to control DC motor is by using pulse width modulation (PWM) technique.
There are tons of specialized chips dedicated for DC motor control but I think, most of people would chose cheap and widely available 555 timer to build affordable PWM controller.
If you need more info about 555 timer and you want to learn how it works then google it. Here I show fast scrap-built solutions that I've tried and what's worked for me.
This 555 based simplified PWM controller allows to regulate speed of DC motor within range 30-95%.
Switching transistor 2N7000 can handle only 200mA current. For more heavy load you should select MOSFET with proper characteristics.
|555 PWM motor speed control.|
To build fully functional PWM controller with programmable frequency and duty cycle, second 555 timer should be involved.
To simplify design and save PCB precious space it's better to use 556 chip which contains two 555 timers.
|PWM motor control. Improved 555 / 556 PWM controller.|