Am I using the Correct GPIO Pins?

Question

I am having trouble with the GPIO pins on my two raspberry pis. I am working on a project that someone has worked on previously. The person set the same code for the button for both pis. Both pis utilize the code:

import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BCM)
GPIO.setup(21, GPIO.OUT, initial=1)
GPIO.setup(16, GPIO.IN, pull_up_down = GPIO.PUD_UP)
time.sleep(2)

while True:
    if(GPIO.input(16) == 0): print "Hello"
GPIO.cleanup()

When I run this code, the pi on top works; however, the pi on the bottom does not. Am i using the correct GPIO pins for the second? What should the GPIO pins on the bottom be?

Answer 1

You have the button connected to GPIO pin 16 and Ground on the top Pi, and both Pis have GPIO pin 21 connected together. The code you showed is setting GPIO pin 21 as an output (and to HIGH) and setting GPIO 16 to be pulled HIGH by default (on the top Pi). When the button is pressed GPIO pin 16 (on the top Pi) is connected to Ground so the statement:

GPIO.input(16) == 0

is true and 'Hello' is printed.

I'd guess the behaviour you want is for it to set GPIO pin 21 to LOW (instead of printing 'Hello'). That way you are changing the state of the GPIO pin 21 on the bottom Pi (and can have it react).

Change the:

print "Hello"

to:

GPIO.output(21, True)

on the top Pi.

Code for the bottom Pi would look something like this:

import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BCM)
GPIO.setup(21, GPIO.IN, pull_up_down = GPIO.PUD_UP)
time.sleep(2)

while True:
    if(GPIO.input(21) == 0): print "Hello"
GPIO.cleanup()

Answer 2

There should be no reason that the code should not work on the second pi other than system configuration errors. (permissions, wiring, etc.) If you are trying to use GPIO 21 as a one-way signal line, make sure that you are not setting both Pis to output, as that can cause unexpected results as well as possibly damaging the chips.
The GPIO output drivers are far more powerful than the input pullup and pulldown resistors in most devices, and the high drivers can usually exceed the power ratings of the chip's GPIO drivers if you short them to ground by using the output low driver instead of the input pulldown resistor.