Operation Manual

Human-computer interfacing
131
Notes:
LED circuit
Now we know all about data and the Raspberry Pi’s input/output options, let’s get
on with building something!
This is about the simplest circuit you can build to test the GPIO outputs of
a Raspberry Pi. This circuit contains just two components: a 1k resistor and an
LED (Light Emitting Diode). The resistor is used to limit the current that flows
out of the Raspberry Pi and into the LED. If there is too much current, you could
break something!
Note that you need to connect the LED up the correct way round. The flat side of
the LED denotes the negative side; the longer leg denotes the positive side of the
LED. The Gertboard already has LEDs wired up exactly like this on some channels.
The following Python program will let you switch the LED on and off. To complete
this exercise you will need the Raspberry Pi GPIO modules. You can either install
these from the Raspberry Pi SD card or download them from
http://pypi.python.org/pypi/RPi.GPIO/
import RPi.GPIO as GPIO
# set up pin 11 to output
GPIO.setu p(11, GPIO.OUT)
state = False
while 1:
GPIO.output(11, state)
command = input("Press return to switch the LED on/off or
'Q' to quit: ")
if command.strip().upper().startswith('Q'):
break
state = not state
Note that this Python script must be run with superuser privileges (as root). You
can do this by running your program from the command line and putting “sudo” in
front of the command you are typing. For example: “sudo python led.py”.
LED circuit experiment
wiring diagram.
Tip...
Please be careful:
If you use too
much current
then you could
easily break
something!