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====== GPIO Inputs - Button LED Control ======
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In the [[gpio_led_control|GPIO LED Control]] section, we looked at how to crate an output pin on the GPIO and turn it on (3.3v) and of (0v).
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In this section we will look at how to create an input pin, that is a GPIO pin that looks for an input (which will be a voltage). When we talk about voltages, the Raspberry Pi only sees a voltage in terms of an on (3.3v) or off (0v). However, other voltages will fall in to either the on or off state, and roughly speaking, these are:
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    Off =   0v - 1.2v
     On = 1.3v - 3.3v
**Remember, the GPIO pins are 3.3v as they are logic devices, attach more than 3.3v and you might have to buy a new Raspberry Pi!**
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==== Pull Up, Pull Down ====
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As we know, the value of a GPIO pin can be between 0v - 3.3v, this is a bit of a problem, because we need a pin to be at a known value so we don't get false positives, for this reason the Pi GPIO actually has built in Pull Up and Pull down resistors.
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{{:pulluppulldown.png?400|}}
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So for any input that we are waiting for a high signal (3.3v) we will use pull down so it stays a 0v, for any pin we are waiting for a low signal (0v) we will use a pull up so it stays at 3.3v (until a button press makes it low)
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If you are using your Raspberry Pi via SSH or a Remote Desktop session, you need to enable remote access of the GPIO Pins, instructions for this are [[enable_remote_access_to_gpio_pins|here]]:
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From the command line, create a new file for editing:
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    sudo nano LED-Button.py
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** You can download the code {{ :led-button.zip |Here}}: **
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    #import modules
    import RPi.GPIO as GPIO    # This imports the GPIO libarary that allows the use of the GPIO pins,
                               # These libraries are built in to Raspbian.
    
    GPIO.setmode (GPIO.BOARD)  # This sets the GPIO pin numbering. Our LED is connected to Pin 12,
                               # so we can reference it by using BOARD as pin 12. However there is
                               # another option (BCM) where we can reference a pin by it's name, pin
                               # 12 is called GPIO18 (a reference to its place on the chip).
    
    GPIO.setup(11, GPIO.IN, pull_up_down=GPIO.PUD_DOWN) # setup GPIO Pin 11 as an input, and set
                                                        # the resistor to Pull Down (PUD_DOWN)
                                                        # this is the pin the button is connected to
                                                        # button is connected from pin 11 to the
                                                        # +3.3v pin on the GPIO
    
    GPIO.setup(12, GPIO.OUT)   # Sets the GPIO pin as output. This is connected to the LED, then
                               # from the LED to 0v via a 330 Ohm resistor.
    
    GPIO.output(12, 0)         # sets the GPIO Pin 12 to low (so 0v)
    
    try:
            while True:                             # start a loop
                    if (GPIO.input(11) == 0):       # if GPIO pin 11 is a 0 (Low (0v)) then..
                            GPIO.output(12,0)       # set Pin 12 to 0v (LED Stays off)
                    else:                           # if GPIO pin is anything other than High (3.3v) then..
                            GPIO.output(12,1)       # set Pin 12 to 3.3v (LED comes on)
    except KeyboardInterrupt:                       # if Ctrl-C is pressed, exit loop
            GPIO.cleanup()                          # reset GPIO pins to default state
    
    #End
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==== The Button in Action ====
Here is the button turning on the LED {{:led-button.mp4|950x574|autoplay,loop}}

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==== A Small Issue ====
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While this code works without any real problems, because we are in a constant loop, this code is very heavy on CPU load, on the Pi Zero I am using this causes the processer to sit at 100 %, a better method is to use an **interrupt**
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This guide is comprised of information from a youtube video by **Gaven MacDonald**. Thanks Gaven.
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