AN INTRODUCTION TO GPIO AND PHYSICAL COMPUTING ON THE RASPBERRY PI
One powerful feature of the Raspberry Pi is the row of GPIO
(general purpose input/output) pins along the edge of the board, next to the
yellow video out socket.
These pins are a physical interface between the Pi and the
outside world. At the simplest level, you can think of them as switches that
you can turn on or off (input) or that the Pi can turn on or off (output).
Seventeen of the 26 pins are GPIO pins; the others are power or ground pins.
WHAT ARE THEY FOR? WHAT
CAN I DO WITH THEM?
You can program the pins to interact
in amazing ways with the real world. Inputs don't have to come from a physical
switch; it could be input from a sensor or a signal from another computer or
device, for example. The output can also do anything, from turning on an LED to
sending a signal or data to another device. If the Raspberry Pi is on a
network, you can control devices that are attached to it from anywhere** and
those devices can send data back. Connectivity and control of physical devices
over the internet is a powerful and exciting thing, and the Raspberry Pi is
ideal for this. There are lots of brilliant examples of physical computing on our
blog.
Note: Not literally anywhere, of course. You need things
like access to the network, a network capable computing device, and
electricity. Please do not write to us to point this out. :)
HOW THE GPIO PINS WORK
OUTPUT
WARNING: If you follow the instructions,
then messing about with the GPIO is safe and fun. Randomly plugging wires and
power sources into your Pi, however, may kill it. Bad things can also happen if
you try to connect things to your Pi that use a lot of power; LEDs are fine,
motors are not. If you are worried about this, then you might want to consider
using a breakout board such as the Pibrella until you are confident enough to use
the GPIO directly.
Ignoring the Pi for a moment, one of
the simplest electrical circuits that you can build is a battery connected to a
light source and a switch (the resistor is there to protect the LED):
When we use a GPIO pin as an output, the Raspberry Pi
replaces both the switch and the battery in the above diagram. Each pin can
turn on or off,or go HIGH or LOW in computing terms. When the pin is HIGH it
outputs 3.3 volts (3v3); when the pin is LOW it is off.
Here's the same circuit using the
Raspberry Pi. The LED is connected to a GPIO pin (which can output +3v3) and a
ground pin (which is 0v and acts like the negative terminal of the battery):
The next step is to write a program to tell the pin to go
HIGH or LOW. Here's an example using Python (see Step 2), and here's how to do it in Scratch.
INPUT
GPIO outputs are easy; they are on or off, HIGH or
LOW, 3v3 or 0v. Inputs are a bit trickier because of the way
that digital devices work. Although it might seem reasonable just to connect a
button across an input pin and a ground pin, the Pi can get confused as to
whether the button is on or off. It might work properly, it might not. It's a
bit like floating about in deep space; without a reference it would be hard to
tell if you were going up or down, or even what up or down meant!
This is why you will see phrases like
"pull up" and "pull down" in Raspberry Pi GPIO tutorials.
It's a way of giving the input pin a reference so it knows for certain when an
input is received.
If you'd like to have a go at using
the GPIO as an input then have a look at ourburping jelly baby and quick reaction game tutorials for Python, or a reaction game for Scratch.
THE END OF THE GUIDE. THE START OF
SOMETHING AMAZING
We hope that this has encouraged you
to have a go at physical computing using the Pi's GPIO; it's really not as
daunting as it looks. It all starts with a simple LED, but it can take you to
incredible places. Do not underestimate the fun, creativity and sense of
achievement you can get from a little computer and a bunch of pins. Have fun!
And if you do make something cool please let us know. :)
GLOSSARY
GPIO
General purpose input/output; in this
specific case the pins on the Raspberry Pi and what you can do with them. So
called because you can use them for all sorts of purposes; most can be used as
either inputs or outputs, depending on your program.
LED
Light-emitting diode- a small,
low-power light source used widely in electronics. Ideal as an introduction to
physical computing on the Pi.
PHYSICAL COMPUTING
Computing that involves tangible
things connected to a computer, beyond standard input and output devices like
keyboards and monitors. Think buttons, lights, robots, alarms, sensors, home
automation, teddy bears called Babbage in near space and so on. We love
physical computing because as well as being lots of fun, it's such a powerful
teaching and learning tool and encourages creativity, problem solving, and
collaboration. Computing beyond the screen engages children of all ages, and you
can make very cool stuff!
APPENDIX 1. A NOTE ON PIN NUMBERING
When programming the GPIO pins there
are two different ways to refer to them: GPIO numbering and physical numbering.
GPIO NUMBERING
These are the GPIO pins as the
computer sees them. The numbers don't make any sense to humans, they jump about
all over the place, so there is no easy way to remember them. You will need a
printed reference or a reference board that fits over the pins.
PHYSICAL NUMBERING
The other way to refer to the pins is
by simply counting across and down from pin 1 at the top left (nearest to the
SD card). This is 'physical numbering' and it looks like this:
WHICH SYSTEM SHOULD I USE?
Beginners and young children may find
the physical numbering system simpler -- you simply count the pins. You'll
still need a diagram like the one above to know which are GPIO pins, which are
ground and which are power though.
Generally we recommend using the GPIO
numbering. It's good practice and most resources use this system. Take your
pick though -- as long as you use the same system within a program then all
will be well. Note that pin numbering can also depend on what programming
language you are using: Scratch GPIO, for example, uses physical pin numbers
whereas in Python you can choose which to use.
For more details on the advanced
capabilities of the GPIO pins see gadgetoid's interactive pinout diagram.
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