This article should acquaint you with basic Arduino programming and show you how to write programs that interact with objects in the real world. (A mandatory disclaimer: the last time I really studied electronics was way back in high school, so this article focuses more on the programming aspects, rather than the electronic side of things.)
Before I start talking about this really cool thing called Arduino (Italian for "good friend"), let me say a few things about the fascinating subject of physical computing. Physical computing has been defined in various ways, but the central idea seems to be the same: physical computing is concerned with developing software that interacts with the world beyond the host computer through a combination of hardware and software—it's aware of the world, so to speak. Such awareness makes these applications capable of sensing external events and responding to them in a predefined way. This is accomplished by the use of sensors and actuators (which I describe next).
The Arduino interacts with the world through actuators and sensors. Sensors are electronic components that describe the world to your application. One common way sensors work is that their electrical properties change (in a mathematically known way) as an effect of the changes in the conditions in which it's operating. For example, the resistance of a photo-resistor changes when the intensity of light incident on it changes. Thermistors are another example of such sensors whose resistance changes when the operating temperature changes. A flex sensor is a different category of sensor, where the resistance changes depending on the extent of the flex or "bend". Such changes can be read as electrical signals on the Arduino's input pin. Depending on the kind of sensor, the signal either can be digital (on or off) or analog (a continuous stream of values). The latter part of this article shows how to work with analog sensors.
Actuators, on the other hand, are electronic components that are used to react to an external event. For example, when it gets dark, the light should be switched on. Sensors and actuators, thus, are used to achieve complementary objectives: one senses, and the other reacts. Examples of actuators are solenoids and servos. Later in this article, I explain how to control a servo using Arduino.
The Arduino is an open-source electronics prototyping platform composed of two major parts: the Arduino board (hardware) and the Arduino IDE (software). The Arduino IDE is used to write the program that will interact with your Arduino and the devices connected to it. In the Arduino world, such a program is called a sketch, which has its origin in its mother language, Processing (see Resources).
The Arduino board is a small-form microcontroller circuit board. At the time of this writing, a number of Arduino boards exist: Arduino UNO, Nano, Mega, Mini, Pro and others (see Resources for a complete list). The Arduino UNO (Figure 1) is the latest version of the basic Arduino board, and you need one of these to follow this article (see Resources for the UNO's detailed specifications).
Figure 1. Arduino UNO (Courtesy of http://arduino.cc/en/Main/ArduinoBoardUno)
Besides the UNO, you need the following hardware to work through this article:
Breadboard to set up the circuit.
Some LEDs.
Resistors: 330 Ohm (at least as many as LEDs), 10 kOhm resistors.
Continuous rotation servo (SpringRC SM-S4303R continuous rotation servo: http://www.robotgear.com.au/Product.aspx/Details/482).
Flex sensor.
Linear potentiometer.
Connecting wires.
One excellent way to get started with Arduino is the Arduino starter kit from Sparkfun. This starter kit contains all the hardware and more that you need to follow this article (except the servo).
If you haven't already opened up your Arduino and plugged it in to your USB port, plug it in. For the purposes of this article, it will be sufficient to use the power supply via the USB connection. If you connect more devices, you will need to connect an external supply.
You will program the Arduino in a language that looks very similar to C and is based on Processing. You can download the Arduino IDE from the Arduino Project Web site.
As you might guess, the IDE is as always the front end. The real pieces are the compilers, linkers and libraries that need to be present to communicate and program the AVR microcontroller-based Arduino. Depending on your Linux distribution, the exact names of the packages will vary, so I just list the software by name here:
The GNU C and C++ compiler for AVR.
AVR binutils.
AVR libc.
avrdude (a program for uploading code to the microcontroller board).
rxtx (for serial communication).
Once these packages are installed, fire up your Arduino IDE. Take a moment to explore the IDE. The buttons for compiling (verifying) and uploading the sketch are the important ones.
The communication between your computer and the Arduino will be via the USB cable that has been packaged with your Arduino board. Once you plug the USB cable in to your computer (with the other end plugged in to the Arduino board), it should show up in the Arduino IDE under Tools?Serial Port as /dev/ttyACMx. If you have more than one USB serial device communicated, be careful to select the correct one. You need to set up user permissions correctly to access the serial port (see Resources for distribution-specific instructions).
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