Can raspberry pi 2 take more than one input

Question

This is basically a followup question to my previous questions (you don't need to read them to answer it). Primary question is, can a Raspberry Pi 2 take more than one inputs from different circuits at the same time (same code running)? I have built a functioning photogate.
Here is my circuit:

The code that I am using is:

import RPi.GPIO as GPIO import time     GPIO.setmode(GPIO.BOARD)
GPIO.setup(11,GPIO.IN)     start = time.time() stop = time.time()
gateState = False     try:
        while True:
            if (GPIO.input(11) != gateState):
                gateState = not gateState                    
                if (gateState == True):
                    start = time.time()    
                else:
                    stop = time.time()
                    if stop - start > 0.0001:
                        print "Time:  ", stop - start, "s"
                        print "Speed: ", 1.5/(stop-start), "m/s"
                        print " " except KeyboardInterrupt:
    GPIO.cleanup()

So, I was wondering, how I would use another photogate on the same Pi at the same time as the first position to measure speed of an object at two times. (basically use two photogates). How would I manipulate the code to use two photogates at the same time, that is if Pi can do it. Which GPIO would I use, (any random two?)

Answer 1

I gave you code in raspberry pi2 photogate which will work with multiple gates and +/- 10 µs timing.

You should be able to see that the code you posted above can not work with multiple gates. In an attempt to gain timing resolution the code you posted is using a busy wait. You can not busy wait in more than one place at the same time.

Answer 2

Yes, you can use multiple input circuits driving multiple GPIOs, provided that the voltage range is appropriate and all share a common ground with the pi.

No, this is not a particularly good way to do time measurements, as the pi typically runs a multistasking operating system such as Linux (or, if you must, Windows) and that is notorious for resulting in a substantial and variable latency in responding to external stimulus. In theory you could overcome this by doing the timing in the kernel, or better yet using a realtime kernel - but it is a lot of effort for marginal results when the real issue is that the wrong platform has been chosen.

Theorectically, if you wanted to do it in the simplest way anyway you could use anotherif checking the other photogate input, or better yet use something blocking on a read of the interrupt node for that gpio, and hope that this results in the scheduler waking you up as quickly as possible. But it will only approximate the timing.

If you want to build a good photogate timer, use a barebones microcontroller and/or feed your inputs directly into a counter timer block's capture trigger. This could potentially be a $1 class part accessorizing the pi, if you really need the capabilities of that for other purposes. (Or to put it in hobbyist language, this is properly an Arduino problem not a pi problem, but you could potentially use a baby Arduino to accomplish it and report the time difference to a pi).