Tuesday, 10 July 2012

RasPi GPIO outputs

Today I decided to give my RasPi GPIO a try. I picked up a breadboard kit ($12.99) and 3 packages of 5 LEDs to (red, green, yellow) each $1.00 and 2 packs of 10 resistors (120 ohm) also each $1.00. The LEDs are 2.1-2.8 voltage drop, leaving about 1.2 volt remaining when being supplied from the 3.3v source of the RasPi (3.3-2.1v assuming minimum voltage drop). Doing the math, in order to keep my current around 10 mA we do V=IR, so 1.2v/0.010A = 120 ohm.

First thing was to install the Python compiler. I didn't do this at first and when trying to compile the RasPi GPIO library I got an error (said it couldn't find Python.h)! Looking to the forums I found the following command to install Python:

sudo apt-get install python-dev

Then according to our trusty friends at TheMagPi issue #2 (page 13), I downloaded the RPi GPIO file using the Midori browser from http://pypi.python.org/pypi/RPi.GPIO (to my home/pi directory) and then used these commands to unzip, untar and then compile it (use sudo if needed):

gunzip RPi.GPIO-0.3.0a.tar.gz
tar -xvf RPi.GPIO-0.3.0a.tar
cd RPi.GPIO-0.3.0a
sudo python setup.py install

Then after a few minutes we have it all installed. Then using nano editor "nano mytest.py", I created my first python program to turn on the GPIO 11 as follows:

import time
import RPi.GPIO as GPIO
# to use Raspberry Pi board pin numbers

# set up the GPIO channels - one output
GPIO.setup(11, GPIO.OUT)
count = 0
while (count < 10):
print "count", count
GPIO.output(11, GPIO.HIGH)
GPIO.output(11, GPIO.LOW)
count = count + 1

Running the program was basically just running "sudo python mytest.py". I hooked up the GPIO 11 pin 11 from my RasPi to the + rail on my breadboard, and had a resistor and LED in series, then ground to pin 6. When the GPIO is set to high it output 3.3v which was enough to turn on the LED. Success!

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