Read a Push Button

At the left is the schematic for a circuit to read a switch. 

Why all the resistors?

• The 50K Ohm resistor is called a pull up resistor.  it ensures that the Pi input pin is normally connected to 3.3 Volts.  This ensures 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.   Because the resistance is so high no significant current is flowing.  0.066 Milli Amps
  

• 1K Ohm resistor between the switch and the ground is  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.
• You may 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

About Prototype (breadboard) Connections

Connections to the Pi

• Yellow - Pin 39: ground
• Red - Pin 40: GPIO 21, Led output
  
• Orange - Pin 38: GPIO 20, Switch input
  
• Green - Pin 1: +3.3 Volts
  

And the code to read the switch.

• When you run this program, you will note that when you push the button the it prints 0 and when you release it prints 1.    Backwards?  

Reference

A game

Craig has created a "Reaction Time Game on the Pi" that you can play with:

You can download it into your current directory with the following command:

• wget "https://raw.githubusercontent.com/cvmiller/react/master/react.py"

Homework

1) Turn the LED on and off with the switch using Python

Yes you could just wire up the switch so that the LED would go on when the switch is pressed. But where's the fun in that?

You have already done most of the programming to accomplish this. You just need to add two programs together and stir a bit.

Steps

• Loop forever(or for a while)
• Read the switch
  • if on turn on the LED
  • if off turn off the led

Use the code from the LED session and the switch session

2) Use the switch to change the flash time of the LED

• Start at a flash time of 1/2 second on and 1/2 second off
• each button push adds 1/2 second to the cycle time
• max 3 seconds on and 3 seconds off
• then go back to 1/2 second cycles.

Debounce.

Some method of delaying or eliminating the rapid on/off that happens when a mechanical switch closes or opens until it makes solid contact or solid release.

Interrupts

Rather than polling to see if the button is pressed we can set the program so that when the button is pressed an interrupt is generated causing a declared function (named the callback function) to be executed. The advantage of doing this is that we will still execute the interrupt function even if the rest of the code is busy sleeping. As it well might be doing if the LED cycle is a couple of seconds or more.

More Scary Code

import time
import signal
import os

import RPi.GPIO as GPIO
	
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

# Values
btn = 20       # Pin number for switch input
count = 0.0    # Count of number of button presses


# signal handlder for SIGINT
def sigint_handler(signal, frame):
    shutdown_requested = True
    print("\nCaught SIGINT, Exiting.")
    os._exit(0)

# Define the interrupt function.  All this does in the example
# is print that the button has been pressed.  channel is gpio pin
def interrupt(channel):
    # Use the already defined variable count, not a unique variable for this function.
    # Doing this allows count to increment and the value to be remembered.
    global count

    # Wait for a bit then read the input to check that we did not get here 
    # due to a spurious falling interrupt on a noisy button release. 
    time.sleep(.1)

    # Return and ignore the interrupt if the gpio value is 1 e.g. button released.
    if GPIO.input(btn) == 1:
      return
    print ("button on input ", channel, " pushed. count=", count)
    count = count + 1.0


# Set up Switch as input with pull up resistor.
GPIO.setup(btn, GPIO.IN, GPIO.PUD_UP)

# Set up interrupt detection.  
#    Falling means this will happen when the input pin goes from 3.3V to ground
#    Bouncetime=300 means that when the interrupt happens, further button
#      interrupts will be ignored for 300ms.  This allows the button to settle.
GPIO.add_event_detect(btn, GPIO.FALLING, callback=interrupt, bouncetime=200)  
 
# register SIGINT signal handler
signal.signal(signal.SIGINT, sigint_handler)

# Carry on forever doing nothing in this example. 
# We could put the LED flashing code here.
while True:
    time.sleep(.01)