a trivial Presence detector with raspberry using python

Question

I want to build a presence detector using raspberry pi, to say if someone is present in a room or not.

So far, the principle is very simple and basic : I use a pir detector to detect movement. After this first step, I want to use a led (for example) which will be red if the room is full and green if it is free. I don't know what I can do after that, but I want to succed in that first. Using the net, I wrote this program (which works ) :

import RPi.GPIO as GPIO
import time
import urllib

GPIO.setmode(GPIO.BCM)

GPIO_PIR = 7

GPIO.setup(GPIO_PIR,GPIO.IN)

Current_State  = 0
Previous_State = 0

try:
    print "Attente detection..."
    while GPIO.input(GPIO_PIR)==1:
        Current_State = 0
    print " Pret"

    while True :

        Current_State = GPIO.input(GPIO_PIR)

        if Current_State==1 and Previous_State==0:
            print " Mouvement detecte !"
            time.sleep(5)
            Previous_State=1
        elif Current_State==0 and Previous_State==1:
            print " Pret "
            Previous_State=0

            time.sleep(1)

except KeyboardInterrupt:
    print " Quit"
GPIO.cleanup()

What I want to do now is to have a message which will contain the state of the room, so this message can be "room full" or "room empty".

For example, if a movement is detected (the terminal will print "movement detected" and after 5 seconds "ready for detection") the message should be "room full". And if no movement has been detected after 10 seconds, the message will switch to "room empty" etc.

That's it ! I know that it is very simple and basic to do it in python (it's not a raspberry question) but I'm not really familiar with python and I don't know how to using that with all this blocks of "if" and "while". Can you help me please to fix that, thank you

Answer 1

You're so close!

Let's jump to what's correct first. In your second while True block, your code already sleeps (waits) for an interval before continuing. Here it is with some comments and formatting corrections:

while True:

    # this reads from your GPIO to set a variable
    Current_State = GPIO.input(GPIO_PIR)

    # if it detected motion or if there wasn't motion last time we checked..
    if Current_State==1 and Previous_State==0:
        print " Mouvement detecte !"

        # wait five seconds so we're not checking as fast as the cpu
        # will allow!
        time.sleep(5)
        Previous_State=1

    # this block has the same logic as above, but in reverse!
    elif Current_State==0 and Previous_State==1:
        # if we don't detect motion on gpio, print ready
        # this is where we need to keep track of how many times we didn't
        # detect motion.
        print " Pret "
        Previous_State=0

        time.sleep(1)

Now, let's make this work. You likely don't want the first while GPIO.input(GPIO_PIR)==1: block as it's going to just block the thread, setting Current_State, even though we just redefine it later (this is also likely blocking your program to getting to the actual While True: loop that does our work.

Here's what a cleaned up version that implements the logic you want looks like:

import RPi.GPIO as GPIO
import time
import urllib

GPIO.setmode(GPIO.BCM)
GPIO_PIR = 7
GPIO.setup(GPIO_PIR,GPIO.IN)

Previous_State = 0
Pret_Counter = 0
pret_message = None

try:
    # this will only print the first time.
    print "Attente detection..."

    # this loop will continuously run
    while True:

        Current_State = GPIO.input(GPIO_PIR)

        # if we have a message, print it!
        if pret_message:
            print pret_message

        if Current_State and Previous_State==0:
            print "Mouvement detecte!"
            time.sleep(5)
            Previous_State=1
            # Someone moved.  reset detection counter.
            Pret_Counter = 0

        elif Pret_Counter > 9:
            # if we've been empty for 10 seconds,
            # don't bother incrementing our counter
            pret_message = "Room empty"

        elif Current_State==0 and Previous_State:
            print "Pret"
            Pret_Counter += 1
            Previous_State=0
            time.sleep(1)

except KeyboardInterrupt:
    print "Quit"
GPIO.cleanup()

I don't have a raspberry pi on hand to test the behavior of the GPIO or the pir detector, but this should do the trick.

Also, You probably want to play with the thresholds a bit -- as your code is now, you check for motion only once every 5 seconds. If no motion is detected twice, you're marking the room as empty. I'd suggest using a similar trick to your new empty logic -- check every 2 seconds maybe 10 times (sometimes meetings are boring and people take micronaps) before deciding it's empty.

As a side note, you should go through a Python tutorial, such as the official Python 2 version if you want to keep learning an old version, or the Python 3 version to learn the current state of Python programming.