• Digital Input
• The Pi GPIO pins can be set to either output or input. So far we have seen output which we used to turn a led on and off. 
  
  
• For input we need to set the desired pin to be an input and set a pull up resistor on.  The purpose of the pull up resistor is to ensure that the input is not floating.  If it were left floating then random environmental electrical noise could cause the input to go from 0 to 1, like, randomly.  
  
  This is not desirable. 
  
  We could wire our switch circuit with a 50K Ohm resistor as seen on the right above.  With the Pi, we can just turn on the internal pull up resistor, diagram to the right. https://en.wikipedia.org/wiki/Pull-up_resistor
  
  

  

• It is also be a good idea to put a 1K Ohm resistor between the switch and the ground in case we accidentally set the pin to output rather than input.  This will limit the output current in case the pin is set to output and the switch closed.
  
  
• The switch supplied is a double pole single throw (DPST) switch.  https://en.wikipedia.org/wiki/Switch
  This means that it has two sets of contacts that close at the same time.
  
  We are only interested in one set and will ignore the other set.   Plug the switch in as shown in the image at the below  This connects the pins from  row 27 to row 29 when the switch is pushed.
  
  

• Once the switch is on the board, wire it up using the jumpers and a resistor from your kit.  Row 29 to the 1K Ohm resistor then to ground and row 16 to GPIO pin 6.  We are going to use pin 6 for input.
  
  
• You need to debounce the input from a switch with some logic or delays.  As the switch closes there is a period when it goes from open to closed a few times before it closes solidly.  The easiest way to do this is to delay a few milliseconds before using a switch value.  https://en.wikipedia.org/wiki/Switch#Contact_bounce
  
  A simple Python program that reads a switch
  
  #! /usr/bin/python
  
  import time
  import RPi.GPIO as GPIO
  GPIO.setmode(GPIO.BCM)
  GPIO.setwarnings(False)
  
  led = 19
  btn = 6
  
  # Set up leds as output
  GPIO.setup(led, GPIO.OUT)
  
  # Set up Switch as input with pull up resistor.
   GPIO.setup(btn, GPIO.IN, GPIO.PUD_UP)
  
  # Set the last input to
  lastInput = 1
  
  # Carry on forever
  while True:
      input = GPIO.input(btn)
      time.sleep(.5)
      print input
  
  
  
  
  
• When you run this program, you will note that when you push the button the light goes off and when you release it it goes back on again.  This is not the way we would normally expect buttons to work, but if you look at the circuit it will make sense.  The pullup resistor holds the Pi input pin high/true/on/1 until the switch is closed.  When the switch is closed the Pi input is pulled to ground or low/false/off/0.  Of course if this is bothering you then you can just change the software.  Change  print input to print not input
  
      input = GPIO.input(btn)
      time.sleep(.5)
      print not input
  
  A simple example of it can all be done with code.
  
  Interesting - note that the print display changed.
  
  
• Add control of the led.
     
      time.sleep(.5)
      input = GPIO.input(btn)
      print not input
      GPIO.output(led, not input)