Ndtyjky

I am writing python code on raspberry pi 3. I am registering an event on input channel 21, to check moisture detection. I am getting this error Runtime error:Failed to add edge detection.
My code is:

import RPi.GPIO as GPIO

import sys,os

import time

import datetime





channel = 21

led_output = 18

GPIO.setmode(GPIO.BCM)

GPIO.setwarnings(False)

GPIO.setup(channel, GPIO.IN)

GPIO.setup(led_output, GPIO.OUT)





def callback(channel):

    filehandle = open("output.txt", "w") or die ("Could not write out")

    if GPIO.input(channel) == 1:

        print ("Water Detected!")

        filehandle.write("1")

        GPIO.output(led_output, GPIO.LOW)

    else:

        print ("Water Not Detected!")

        filehandle.write("0")

        GPIO.output(led_output, GPIO.HIGH)

    filehandle.close()









 GPIO.add_event_detect(channel, GPIO.BOTH, bouncetime=300)   

 GPIO.add_event_callback(channel, callback)  



    print(GPIO.input(channel))



    while True:

        time.sleep(5)

When I reboot the Raspberry and run your code it works perfect.
Only after killing the process or CTRL-C keyboard interrupting and running it again the problem/error occurs. I think this has to do with the fact that you exit the program without cleaning up properly...
I got it working in case you exit the running the program with CTRL-C with the code below in which I included a GPIO.cleanup()
However...this unfortunately it does not cover the situation in which you simply kill the running programm...In that case you still need to reboot.
So there is room for improvement.
Please re-insert your own file management commands again.

import RPi.GPIO as GPIO  

import sys,os

import time

import datetime



channel = 21

led_output = 18

GPIO.setmode(GPIO.BCM)

GPIO.setwarnings(False)



GPIO.setup(channel, GPIO.IN)

GPIO.setup(led_output, GPIO.OUT)



def callback(channel):

    if GPIO.input(channel) == 1:

        print ("Water Detected!")

        GPIO.output(led_output, GPIO.LOW)

    else:

        print ("Water Not Detected!")

        GPIO.output(led_output, GPIO.HIGH)



GPIO.add_event_detect(channel, GPIO.BOTH, bouncetime=300)   

GPIO.add_event_callback(channel, callback)  

print(GPIO.input(channel))





try:

    while True:

        #main loop here with some (dummy) code

        eg_set_a_dummy_variable = 0



except KeyboardInterrupt:  

    # here you put any code you want to run before the program   

    # exits when you press CTRL+C  

    print ("Program interrupted by CTRL-C")



except:  

    # this catches ALL other exceptions including errors.  

    # You won't get any error messages for debugging  

    # so only use it once your code is working  

    print ("Other error or exception occurred!") 



finally:

     # this ensures a clean exit and prevents your

     # error "Runtime error:Failed to add edge detection"

     # in case you run your code for the second time after a break

    GPIO.cleanup()



     # credits to:

     # https://raspi.tv/2013/rpi-gpio-basics-3-how-to-exit-gpio-programs-cleanly-avoid-warnings-and-protect-your-pi

It is not very clean solution, but you can call GPIO.cleanup() at the start of your script too for case when your process was killed before and GPIO.cleanup() was not called.