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Getting Started with Raspberry Pi GPIO and Python

Wall's Corners

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Title: Getting Started with Raspberry Pi GPIO and Python
Author: Wall's Corners
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1 Overview This is the first of two articles showing basic GPIO on the Raspberry-Pi using the prototype area of the Slice of Pi. This c...

<h2>1 Overview</h2>
<p> </p>
<p>This is the first of two articles showing basic <a href="http://bit.ly/26nCnnf" itemprop="url" target="_bank" title="GPIO"><strong>GPIO</strong></a> on the Raspberry-Pi using the prototype area of the Slice of Pi. This covers basic details on the GPIO pins, setting up a <a href="http://bit.ly/1pxf1tx" itemprop="url" target="_bank" title="Python"><strong>Python</strong></a> library to allow access to the GPIO. There is an example circuit to build on the Slice and some code to get the outputs working.<br />
This was originally a blog post on Matts blog at <a href="http://bit.ly/1WT6QW6" target="_blank">lwk.mjhosting.co.uk</a></p>
<p> </p>
<h2><a name="2"></a>2 GPIO Basic’s</h2>
<p><a href="http://bit.ly/1qUAs90" target="_blank"><img alt="" height="225" src="http://bit.ly/1WT6T4m" title="R-Pi" width="300" /></a><br />
The R-Pi has 17 GPIO pins brought out onto the header, most have alternated functions other than just I/O, there are two pins for UART, two for <a href="http://bit.ly/1qUAs94" itemprop="url" target="_bank" title="I2C"><strong>I2C</strong></a> and six for SPI. All the pins can be use for GPIO with either INPUT or OUTPUT, there also internal pull-up & pull-downs for each pin but the I2C pins have and onboard pull-up so using them for GPIO may not work in some cases.</p>
<p>Using any of the pins will require extra care, than most <a href="http://bit.ly/26nCm2w" itemprop="url" target="_bank" title="Arduino"><strong>Arduino</strong></a> users maybe be used to. These pins are 3V3 not 5V like the AVR chips, and they a directly connected to the Broadcom chip at the heart of the R-Pi. This means there is not protection, if you send 5V down a pin there is a good chance of killing the Pi.</p>
<p>There will also be an issue with trying to draw to much power form the pins, according to the data-sheet each pin programmed to current drive between 2mA and 16mA, and it has been warned that trying to draw 16mA from several pins at once could also lead to a damaged Pi.</p>
<p>Also from the <a href="http://bit.ly/1qUAu0F" target="_blank" title="R-Pi Wiki">wiki</a> the “maximum permitted current draw from the 3v3 pin is 50mA” and the “maximum permitted current draw from the 5v pin is the USB input current (usually 1A) minus any current draw from the rest of the board.” The current draw for Model B is stated as 700mA so with a 1A power supply this leaves about 300mA to play with.</p>
<p>Anyway enough about power theres still plenty to try driving some basis LED’s and use the UART to talk to an XRF.</p>
<p>So how does one go about talking to the GPIO’s? Well thanks to a little <a href="http://bit.ly/1WT6QWb" itemprop="url" target="_bank" title="Controlling DC Motors Using Python With a Raspberry Pi"><strong>python</strong></a> library its nice and simple. I’ll be using a Debian install but this should work on the others.</p>
<p> </p>
<h2><a name="3"></a>3 Installing RPi.GPIO</h2>
<p><em>NOTE 1: The instructions here refer to an early version of RPi.GPIO. Please search the web for the latest version and replace the version numbers in the instructions below.</em></p>
<p><em>NOTE 2: If you have purchased Ciseco’s <a href="http://bit.ly/1qUAs99" target="_blank">4G Raspberry Wheezy SD card, configured for Ciseco products</a></em><em>, or used the Ciseco Wheezy image to create an SD card yourself, then the RPi.GPIO should already be installed and you can skip this step. </em></p>
<p><a href="http://bit.ly/1WT6TkB" target="_blank" title="RPi.GPIO">RPi.GPIO</a> is a small python library that take some of the complexity out of driving the GPIO pins, once install a single LED can be lit with 3 lines of python. Installing the library is almost as simple, either at a text console or using LXTerminal enter the following</p>
<p> </p>
<pre>
$ wget http://bit.ly/1qUAspo
    $ tar zxf RPi.GPIO-0.1.0.tar.gz
    $ cd RPi.GPIO-0.1.0
    $ sudo python setup.py install
</pre>
<p><a href="http://bit.ly/1WT6TkC" target="_blank"><img alt="" height="474" src="http://bit.ly/1WT6TkC" title="RPi.GPIO setup" width="672" /></a></p>
<p>It should look something like the above just before you hit enter on the last command. That’s it we now have RPi.GPIO python library installed.</p>
<p> </p>
<p> </p>
<h2><a name="4"></a>4 LED and Pushbutton Slice</h2>
<p>So now I need and LED or two to Flash. So using a <a href="http://bit.ly/1qUAuh1" target="_blank" title="Slice of Pi">Slice of Pi</a> from <a href="http://bit.ly/1WT6TkE" target="_blank" title="Ciseco PLC">Ciseco</a> I have wired up four LED’s with resistors and two Push button’s.</p>
<p>First to be soldered up were the LED’s and push buttons then I adde the resistors and connecting wires.</p>
<p>To keep the GPIO pins nice and safe I’m using 470R resistors for the RED,YELLOW and GREEN LED’s and a 330R for the BLUE, this will keep the current of each led to around 6-7mA. Here’s the schematic and the board</p>
<p><a href="http://bit.ly/1qUAspt" target="_blank"><img alt="" height="225" src="http://bit.ly/1WT6QWc" title="Slice Schematic 1" width="300" /></a> <a href="http://bit.ly/1qUAuh6" target="_blank"><img alt="" height="300" src="http://bit.ly/1WT6QWe" title="IMG_7468" width="225" /></a></p>
<p>If you look at the LED’s I’m using the 3.3v rail to power the led and have the cathode going to the GPIO’s this mean that to turn the LED on we set the output to LOW or False (in python’s case) but to turn the off we set the output to HIGH or True.<br />
This means we a sinking the current through the Raspberry Pi rather that trying to source it from the pin’s. For the push button we are using a 10K pull-down resistor, this makes sure the button read a solid LOW or False when not pressed, when pressed the 3.3v is connected and we get a solid HIGH or True reading.</p>
<p> </p>
<h2><a name="5"></a>5 Some Code</h2>
<p>I said turning LED’s on was easy well try this in a terminal:</p>
<pre>
$ sudo python
    >>> import RPi.GPIO as GPIO
    >>> GPIO.setup(18, GPIO.OUT)
    >>> GPIO.output(18, False)
</pre>
<p> </p>
<p>Again it should look a little some thing like this</p>
<p><a href="http://bit.ly/1qUAuhb" target="_blank"><img alt="" height="474" src="http://bit.ly/1qUAuhb" title="GPIO Test code" width="672" /></a></p>
<p><a href="http://bit.ly/1WT6Rcs" target="_blank"><img alt="" height="225" src="http://bit.ly/1qUAspv" title="Red LED" width="300" /></a></p>
<p>Anyone trying this themselves may notice that the RED LED actual turned on after the GPIO.setup line and that GPIO.output did nothing. This is because we have the cathode of the LED’s wired to the GPIO, and are using the Raspberry Pi to switch the GND. The default state for a output pin is LOW and so this also power to flow throughout the LED.  So how about turning that yellow LED on:-</p>
<pre>
>>> GPIO.setup(16, GPIO.OUT)
</pre>
<p> </p>
<p><a href="http://bit.ly/1WT6Rcw" target="_blank"><img alt="" height="474" src="http://bit.ly/1WT6Rcw" title="GPIO Code 2" width="672" /></a></p>
<p><a href="http://bit.ly/1qUAspB" target="_blank"><img alt="" height="225" src="http://bit.ly/1WT6Rcy" title="Yellow LED" width="300" /></a></p>
<p>Simple, now to turn both LED’s off we set the inputs HIGH or True like so:</p>
<pre>
>>> GPIO.output(18, True)
    >>> GPIO.output(16, True)
</pre>
<p> </p>
<p>Now why if we wired the red LED to GPIO5 on the Slice of Pi, why am I using 18 to control it? Well theres a bit of fun with the pin numbering on Raspberry Pi, and some how i don’t expect it to go anyway and time soon, it like that odd gap on the <a href="http://bit.ly/1XReOgM" itemprop="url" target="_bank" title="Arduino"><strong>Arduino</strong></a> headers. It comes from this post originally <a href="http://bit.ly/1WT6RcA" target="_blank">Pinout for GPIO connectors</a>, better shown in the left pic bellow, anyway it turns out the original numbering of GPIO0-GPOI7 relates to nothing logical once inside linux, as they pins are accessed via there BCM2835(the chip on the Pi) GPIO numbers. See the right pic for the internal numbering, also all the signal named pins can be used as GPIO’s and have numbers as well.</p>
<p>Anyway RPi.GPIO python library has used yet another form of reference which is to us the pin number on the header, This give three names for each pin <img alt=":(" class="wp-smiley" src="http://bit.ly/1qUAuxA" style="height: 1em; max-height: 1em;" /> When referred to by pin number (officially P1_18) we are talking about pin 18 as counted out on the header, GPIO5 by name and GPIO24 internally by the BCM2835. Now if you ever work with the IO using the Shell file access or C memory mapped registers your going to use the BCM2835 numbering.<br />
Confused yet, I was till did a little table to help. There more on this on the Raspberry Pi wiki page <a href="http://bit.ly/1qUAu0F" target="_blank">Low Level Peripherals</a></p>
<p>For now I will leave you with the diagrams and table bellow, and just know you need to use the RPi.GPIO numbers in python.<br />
This is all ready getting far to long so ill cover the inputs and making something interactive in another post.</p>
<p><a href="http://bit.ly/1WT6TkI" target="_blank"><img alt="" height="586" src="http://bit.ly/1WT6TkI" title="GPIOs" width="254" /></a><a href="http://bit.ly/1qUAsFW" target="_blank"><img alt="" height="586" src="http://bit.ly/1qUAsFW" title="GPIOs-c-n" width="254" /></a></p>
<table border="1">
<tbody>
<tr>
<th>Pin Numbers</th>
<th>RPi.GPIO</th>
<th>Raspberry Pi Name</th>
<th>BCM2835</th>
</tr>
<tr>
<td>P1_01</td>
<td>1</td>
<td>3V3</td>
<td></td>
</tr>
<tr>
<td>P1_02</td>
<td>2</td>
<td>5V0</td>
<td></td>
</tr>
<tr>
<td>P1_03</td>
<td>3</td>
<td>SDA0</td>
<td>GPIO0</td>
</tr>
<tr>
<td>P1_04</td>
<td>4</td>
<td>DNC</td>
<td></td>
</tr>
<tr>
<td>P1_05</td>
<td>5</td>
<td>SCL0</td>
<td>GPIO1</td>
</tr>
<tr>
<td>P1_06</td>
<td>6</td>
<td>GND</td>
<td></td>
</tr>
<tr>
<td>P1_07</td>
<td>7</td>
<td>GPIO7</td>
<td>GPIO4</td>
</tr>
<tr>
<td>P1_08</td>
<td>8</td>
<td>TXD</td>
<td>GPIO14</td>
</tr>
<tr>
<td>P1_09</td>
<td>9</td>
<td>DNC</td>
<td></td>
</tr>
<tr>
<td>P1_10</td>
<td>10</td>
<td>RXD</td>
<td>GPIO15</td>
</tr>
<tr>
<td>P1_11</td>
<td>11</td>
<td>GPIO0</td>
<td>GPIO17</td>
</tr>
<tr>
<td>P1_12</td>
<td>12</td>
<td>GPIO1</td>
<td>GPIO18</td>
</tr>
<tr>
<td>P1_13</td>
<td>13</td>
<td>GPIO2</td>
<td>GPIO21</td>
</tr>
<tr>
<td>P1_14</td>
<td>14</td>
<td>DNC</td>
<td></td>
</tr>
<tr>
<td>P1_15</td>
<td>15</td>
<td>GPIO3</td>
<td>GPIO22</td>
</tr>
<tr>
<td>P1_16</td>
<td>16</td>
<td>GPIO4</td>
<td>GPIO23</td>
</tr>
<tr>
<td>P1_17</td>
<td>17</td>
<td>DNC</td>
<td></td>
</tr>
<tr>
<td>P1_18</td>
<td>18</td>
<td>GPIO5</td>
<td>GPIO24</td>
</tr>
<tr>
<td>P1_19</td>
<td>19</td>
<td>SPI_MOSI</td>
<td>GPIO10</td>
</tr>
<tr>
<td>P1_20</td>
<td>20</td>
<td>DNC</td>
<td></td>
</tr>
<tr>
<td>P1_21</td>
<td>21</td>
<td>SPI_MISO</td>
<td>GPIO9</td>
</tr>
<tr>
<td>P1_22</td>
<td>22</td>
<td>GPIO6</td>
<td>GPIO25</td>
</tr>
<tr>
<td>P1_23</td>
<td>23</td>
<td>SPI_SCLK</td>
<td>GPIO11</td>
</tr>
<tr>
<td>P1_24</td>
<td>24</td>
<td>SPI_CE0_N</td>
<td>GPIO8</td>
</tr>
<tr>
<td>P1_25</td>
<td>25</td>
<td>DNC</td>
<td></td>
</tr>
<tr>
<td>P1_26</td>
<td>26</td>
<td>SPI_CE1_N</td>
<td>GPIO7</td>
</tr>
</tbody>
</table>
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Getting Started with Raspberry Pi GPIO and Python

1 Overview This is the first of two articles showing basic GPIO on the Raspberry-Pi using the prototype area of the Slice of Pi. This covers basic details on the GPIO pins, setting up a Python libra…

27 Apr 2016

Arduino Programming For Beginners: The Traffic Light Controller

Wall's Corners

Link Author

Title: Arduino Programming For Beginners: The Traffic Light Controller
Author: Wall's Corners
Rating 5 of 5 Des:

Last week, we learnt about the basic structure of an Arduino program and took a closer look at the ‘blink’ example. Hopefully you took the ...

<p>Last week, we learnt about the basic structure of an <a href="http://ift.tt/25oS3pH" itemprop="url" target="_bank" title="Arduino"><strong>Arduino</strong></a> program and took a closer look at the ‘blink’ example. Hopefully you took the opportunity to experiment with code, adjusting the timings. This time, we’ll be writing something from scratch. In fact, let’s make a traffic <a href="http://ift.tt/1SDfTER" itemprop="url" target="_bank" title="Create your own musical light show with Raspberry Pi"><strong>light</strong></a>.</p>
<p>Apart from the basic Arduino, you’ll need:</p>
<ul>
<li>A red, yellow and green <a href="http://ift.tt/1Rq2vDU" itemprop="url" target="_bank" title="LED"><strong>LED</strong></a>.</li>
<li>A breadboard.</li>
<li>3 x suitable resistors for the LEDs you have ( probably 220 Ohms is fine).</li>
<li>Connecting wires.</li>
<li>A pushbutton switch.</li>
<li>A high value resistor (10k).</li>
</ul>
<p>You may also want to download and install <a href="http://ift.tt/qNNRif" target="_blank">Fritzing</a>, the tool I used to draw the wiring diagrams for these projects.</p>
<h2>Wiring</h2>
<p>Here’s a quick schematic I made up in Fritzing. It’s very simple – just the three LEDs wired with resistors to three separate input pins, and all connected to the ground.</p>
<p><img alt="traffic light controller" class="aligncenter" height="553" src="http://ift.tt/1WqbQBi" width="416" /></p>
<h2><a href="http://yoursmart.mobi/" target="_blank">Programming</a></h2>
<p>We’ll start by defining variables so that we can address the lights by name rather than a number. Start a new Arduino project, and begin with these lines:</p>
<pre lang="c" xml:lang="c">
int red = 13;
int yellow = 12;
int green = 11;
</pre>
<p>Next, let’s add the setup function, where’ll we define the red, yellow and green LEDs to be output mode. Since we’ve created variables to represent the pin numbers, we can now refer to the pins by names instead.</p>
<pre lang="c" xml:lang="c">
void setup(){
pinMode(red,OUTPUT);
pinMode(yellow,OUTPUT);
pinMode(green,OUTPUT);
}
</pre>
<p>That was easy. Now for the difficult part – the actual logic of a traffic <a href="http://ift.tt/20bGhLI" itemprop="url" target="_bank" title="How to Build an RGB LED Flashlight"><strong>light</strong></a>. I’m going to create a separate function for changing the lights, and you’ll see why later.</p>
<p>When you first begin programming, the code itself is very rudimentary – it’s figuring out the minute logic details that presents the biggest problem. The key to being a good programmer is to be able to look at any process, and break it down into its fundamental steps.</p>
<pre lang="c" xml:lang="c">
void loop(){
changeLights();
delay(15000);
}

void changeLights(){
// green off, yellow for 3 seconds
digitalWrite(green,HIGH);
digitalWrite(yellow,LOW);
delay(3000);

// turn off yellow, then turn red on for 5 seconds
digitalWrite(yellow,LOW);
digitalWrite(red,HIGH);
delay(5000);

// red and yellow on for 2 seconds (red is already on though)
digitalWrite(yellow,HIGH);
delay(2000);

// turn off red and yellow, then turn on green
digitalWrite(yellow,LOW);
digitalWrite(red,LOW);
digitalWrite(green,HIGH);
}
</pre>
<p>Done! Now, upload and run. You should have a working traffic light that changes every 15 seconds. However, I think it’s a little basic right now, so let’s add in a pushbutton for pedestrians to change the light whenever they like. Refer to the updated circuit diagram below:</p>
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<p><img alt="traffic light controller" class="aligncenter" height="674" src="http://ift.tt/1WqbOJC" width="550" /></p>
<p>You’ll notice that the switch has a high-impedance 10k resistor attached to it, and may be wondering why. This is called a pull down resistor. It’s a difficult concept to grasp at first, but bear with me.</p>
<p>A switch either lets the current flow, or doesn’t. This seems simple enough, but in a logic circuit, the current should be always flowing in either a high or low state (remember – 0 or 1, high or low). You might assume that a pushbutton switch that isn’t actually being pushed would be defined as being in a low state, but in fact it’s said to be ‘floating’, because no current is being drawn at all.</p>
<p>In this floating state, it’s possible that a false reading will occur as it fluctuates with electrical interference. In other words, a floating switch is giving neither a reliable high, nor low state reading. A pull down resistor keeps a small amount of current flowing when the switch is closed, thereby ensuring an accurate low state reading. In other logic circuits, you may find a pull-up resistor instead – this works on the same principle, but in reverse, making sure that particular logic gate defaults to high.</p>
<p>Now, in the loop part of the code, instead of changing the lights every 15 seconds, we’re going to read the state of the pushbutton switch instead, and only change the lights when it’s activated.</p>
<p>Start by adding some new variables to the start of the app:</p>
<pre lang="c" xml:lang="c">
int button = 2; // switch is on pin 2
int buttonValue = 0; // switch defaults to 0 or LOW
</pre>
<p>Now, in the setup function, add a new line to declare the switch as an input. I’ve also added a single line to start the traffic lights in the green stage. Without this initial setting, they would be turned off, until the first time a changeLights() was initiated using a function.</p>
<pre lang="c" xml:lang="c">
pinMode(switch,INPUT);
digitalWrite(green,HIGH);
</pre>
<p>Change the entire loop function to the following instead:</p>
<pre lang="c" xml:lang="c">
void loop(){
// read the value of the switch
switchValue = digitalRead(button);
</pre>
<pre lang="c" xml:lang="c">
// if the switch is HIGH, ie. pushed down - change the lights!
if (buttonValue == HIGH){
changeLights();
delay(15000); // wait for 15 seconds
}
}
</pre>
<p>That should do it. By waiting inside the “if” statement for 15 seconds, we ensure the traffic lights can’t change for at least that duration. Once 15 seconds is up, the loop restarts. Each restart of the loop, we will read the state of the button again, but if it isn’t pressed then the “if” statement never activates, the lights never change, and it simply restarts again.</p>
<p>That’s it for this time, so I hope you learnt lots and had fun writing from scratch. You got your hands dirty with a few LEDs and resistors, as well as adding a switch with a pull down resistor – hopefully you understood why we need one. Play around with the code, adjust it, and most importantly have fun. If you make any additions or changes, why not let us know about it in the comments?</p>
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Arduino Programming For Beginners: The Traffic Light Controller

Last week, we learnt about the basic structure of an Arduino program and took a closer look at the ‘blink’ example. Hopefully you took the opportunity to experiment with code, adjusting the timings. T…

12 Apr 2016

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Arduino Programming For Beginners: The Traffic Light Controller

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