Page 185 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 6 Options for Creating and Controlling Motion 163

P roject 6-7: Use Software PWM to Control Speed

We will re-create the hardware PWM project in software so you can get a feel for
what the Arduino can do. This example assumes you’ve downloaded the software,
installed the drivers for your PC or Mac, and identified the port your Arduino is
plugged into. Refer to the “Arduino Primer” section in the appendix for how to set up
the Arduino to communicate with your computer.
Unfortunately, you usually can’t plug motors directly into the Arduino. The Arduino
can source up to only 40mA on each of the input/output pins, and up to 500mA
through the power pins when connected through USB. A lot of motors you’ll use
need more current than this. We get around this issue by using the Arduino to give
the motor instructions through a transistor, and giving the motor a separate power
supply. This is similar to Project 6-6, except we’ll replace the 555 timer with an
Arduino.

In this project, we’ll create a sketch that listens for input from the stuff you have plugged
in (switches, sensors, and so on) and then talks to components you want to control
(such as motors). We’ll build the circuit first, and then go over how to turn a motor
on and off through a transistor with code from the Arduino, and finally use PWM for
speed control through the Arduino. (You can find plenty of well-documented example
sketches of using the Arduino to talk to motors and other components. For example,
see http://arduino.cc/en/Tutorial/HomePage for basic sketches. In most cases. you can
just start with these examples and modify them as you see fit.)

Shopping List:

• Arduino with USB cable

• DC toy motor with wire leads
• Corresponding battery (9V used here) and snap or holder with wire leads (like
RadioShack 270-324)
• Breadboard (like All Electronics PB-400)

• Jumper wires (like SparkFun PRT-00124) or hook-up wire to make your own
(see Project 6-4)