Canopy provides 450+ scientific and analytic Python packages plus an integrated environment for data analysis, visualization & application development. The GPIO pins on a Raspberry Pi are a great way to interface physical devices like buttons and LEDs with the little Linux processor. If you’re a Python developer.

The GPIO pins on a Raspberry Pi are a great way to interface physical devices like buttons and LEDs with the little Linux processor. If you’re a Python developer, there’s a sweet library called RPi.GPIO that handles interfacing with the pins.

In just three lines of code, you can get an LED blinking on one of the GPIO pins. Not sure if Raspberry Pi is right for you? Lets you dial into the field to find the best board for your needs. Installation The newest version of Raspbian has the RPi.GPIO library pre-installed. You’ll probably need to update your library, so using the command line, run: sudo python import RPi.GPIO as GPIO GPIO.VERSION The current version of RPi.GPIO is 0.5.4 If you need to update to a newer version, run: sudo apt-get update sudo apt-get upgrade If you don’t have the RPi.GPIO library because you’re using an older version of Raspbian, there are great instructions on the website on installing the package from scratch. Using the RPi.GPIO Library Now that you’ve got the package installed and updated, let’s take a look at some of the functions that come with it. Open the Leafpad text editor and save your sketch as “myInputSketch.py”.

From this point forward, we’ll execute this script using the command line: sudo python myInputSketch.py All of the following code can be added to this same file. Remember to save before you run the above command. To exit the sketch and make changes, press Ctrl+C.

To add the GPIO library to a Python sketch, you must first import it: import RPi.GPIO as GPIO Then we need to declare the type of numbering system we’re going to use for our pins: #set up GPIO using BCM numbering GPIO.setmode(GPIO.BCM) #setup GPIO using Board numbering GPIO.setmode(GPIO.BOARD) The main difference between these modes is that the BOARD option uses the pins exactly as they are laid out on the Pi. No matter what revision you’re using, these will always be the same. The BCM option uses the Broadcom SoC numbering, which differs between version 1 and version 2 of the Pi. From In the image above, you’ll see that Pin 5 is GPIO01/03. This means that a v.1 Pi is GPIO 01, while a v.2 Pi is GPIO 03. The BCM numbering is what I’ll be using for the rest of this entry, because it’s universal across other programming languages.

Building a Circuit Now we’re going to get into inputs and outputs. In the circuit shown below, two momentary switches are wired to GPIO pins 23 and 24 (pins 16 and 18 on the board). The switch on pin 23 is tied to 3.3V, while the switch on pin 24 is tied to ground. The reason for this is that the Raspberry Pi has internal pull-up and pull-down resistors that can be specified when the pin declarations are made. To set up these pins, write: GPIO.setup(23, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) GPIO.setup(24, GPIO.IN, pull_up_down=GPIO.PUD_UP) This will enable a pull-down resistor on pin 23, and a pull-up resistor on pin 24. Now, let’s check to see if we can read them. The Pi is looking for a high voltage on Pin 23 and a low voltage on Pin 24.

We’ll also need to put these inside of a loop, so that it is constantly checking the pin voltage. The code so far looks like this: import RPi.GPIO as GPIO GPIO.setmode(GPIO.BCM) GPIO.setup(23, GPIO.IN, pull_up_down = GPIO.PUD_DOWN) GPIO.setup(24, GPIO.IN, pull_up_down = GPIO.PUD_UP) while True: if(GPIO.input(23) ==1): print(“Button 1 pressed”) if(GPIO.input(24) == 0): print(“Button 2 pressed”) GPIO.cleanup() The indents in Python are important when using loops, so be sure to include them. You also must run your script as “sudo” to access the GPIO pins. The GPIO.cleanup() command at the end is necessary to reset the status of any GPIO pins when you exit the program.

If you don’t use this, then the GPIO pins will remain at whatever state they were last set to. The Problem With Polling This code works, but prints a line for each frame that the button is pressed. This is extremely inconvenient if you want to use that button to trigger an action or command only one time. Luckily, the GPIO library has built in a rising-edge and falling-edge function. A rising-edge is defined by the time the pin changes from low to high, but it only detects the change.

Similarly, the falling-edge is the moment the pin changes from high to low. Using this definition, let’s change our code slightly: import RPi.GPIO as GPIO GPIO.setmode(GPIO.BCM) GPIO.setup(23, GPIO.IN, pull_up_down = GPIO.PUD_DOWN) GPIO.setup(24, GPIO.IN, pull_up_down = GPIO.PUD_UP) while True: GPIO.wait_for_edge(23, GPIO.RISING) print(“Button 1 Pressed”) GPIO.wait_for_edge(23, GPIO.FALLING) print(“Button 1 Released”) GPIO.wait_for_edge(24, GPIO.FALLING) print(“Button 2 Pressed”) GPIO.wait_for_edge(24, GPIO.RISING) print(“Button 2 Released”) GPIO.cleanup() When you run this code, notice how the statement only runs after the edge detection occurs. This is because Python is waiting for this specific edge to occur before proceeding with the rest of the code. What happens if you try to press button 2 before you let go of button 1?

What happens if you try to press button 1 twice without pressing button 2? Because the code is written sequentially, the edges must occur in exactly the order written.

Edge Detection is great if you need to wait for an input before continuing with the rest of the code. However, if you need to trigger a function using an input device, then events and callback functions are the best way to do that. Events and Callback Functions Let’s say you’ve got the Raspberry Pi camera module, and you’d like it to snap a photo when you press a button. However, you don’t want your code to poll that button constantly, and you certainly don’t want to wait for an edge because you may have other code running simultaneously.

The best way to execute this code is using a callback function. 3800 Useful Chinese Sentences Pdf To Jpg. This is a function that is attached to a specific GPIO pin and run whenever that edge is detected. Great lesson! I followed it exactly, but I am getting “ghosts in the machine” as my Pi is registering button presses when none have occurred, and sometimes fails to see button releases. I have tried various pull up and pull down resistors, and the problem seems more pronounced when triggering the pull down pin.

I have 3 Pi’s around the house and have tried the code with all of them and they all respond the same. By simply touching the wires on the breadboard between the switch and GPIO pins I get input triggers. Any suggestions would be greatly appreciated as I have re-entered the code and tried different components with only slightly different results, but none of them satisfactory. I am hoping to pole a set of contacts on my furnace so I can keep track of how long the furnace is firing, so I can calculate how much heating oil I have used (knowing that the unit consumes.75 gallons an hour, if I can keep track of the total run-time I would have a primitive flow meter of sorts) I will keep tweaking it, but any clues would be helpful. Great write up Mark:) Here is a serious question however.

I’m currently on a project and I would like to capture a temp value from a user who only has a keypad membrane with 4 buttons (each button is connected to it’s own GPIO). I would like to use one of the buttons to scroll through a set or defined number values e.g. 1, 2,, 50 and rotate back to 1 again once they hit the top value (I would also like to include letters and symbols but that’s easy I’m sure once I’ve got the basic number rotation working). Sithara Serial Actress Photos. The goal is to create a kind of like a next, next action until they find the desired value.

Please if you could point me in the right direction that would be awesome. Thanks in advance Brett •.