If you want to build electronic devices, at some point you're going to need to read data through GPIO. In this exercise, we're going to read the value of a push button on a breadboard.
Set up the circuit as shown in the pictures on the right. Note that one end of the 4.7kOhm resistor connected to pin 23 is connected to the positive supply rail. The red jum[er cable links the positive rail on the breadboard to the 3.3V pin on the GPIO header. The green jumper wire connected to one of the pins on the button is connected to the negative rail on the breadboard, and there's another jumper cable linking that negative supply rail to a GND pin on the GPIO header.
The resistor is used to pull up the voltage on pin 23 to logic 1. Without it, pin 23 would have an indeterminate value. When the button is pressed, pin 23 is connected directly to ground, so it switches to logic 0.
Save the following code in a file called button.py
#!/usr/bin/env python import time import RPi.GPIO as GPIO def main(): # tell the GPIO module that we want to use the # chip's pin numbering scheme GPIO.setmode(GPIO.BCM) # setup pin 25 as an output GPIO.setup(23,GPIO.IN) GPIO.setup(24,GPIO.OUT) GPIO.setup(25,GPIO.OUT) GPIO.output(25,True) while True: if GPIO.input(23): # the button is being pressed, so turn on the green LED # and turn off the red LED GPIO.output(24,True) GPIO.output(25,False) print "button true" else: # the button isn't being pressed, so turn off the green LED # and turn on the red LED GPIO.output(24,False) GPIO.output(25,True)
For more detail: Detecting a button press through GPIO