Python GPIO.add_event_detect works the wrong way around

The IR sensors I have come across are active low, i.e. they are normally high but will pulse low and high as they receive an IR signal.

This together with the fact you only get transitions between low and high when a signal is being received explains what you are seeing.

The following script shows the sort of logic you need to add to detect this state. pigpio has a watchdog function which triggers when a GPIO is inactive which makes the logic slightly simpler.

You can test by switching PWM on and off for the GPIO.

E.g. pigs p 4 100 will show active and pigs p 4 0 will show inactive.

#!/usr/bin/env python

# listen.py
# 2018-02-16
# Public Domain

# listen.py    # listen on GPIO 4
# listen.py 23 # listen on GPIO 23

import sys
import time
import pigpio

inactive = False

def cbf(GPIO, level, tick):
   global inactive
   if level == pigpio.TIMEOUT:
      if inactive == False:
         inactive = True
         print("Inactive")
   else:
      if inactive == True:
         inactive = False
         print("Active")

pi = pigpio.pi()

if not pi.connected:
   exit()

if len(sys.argv) == 1:
   G = 4
else:
   G = int(sys.argv[1])

cb = pi.callback(G, pigpio.EITHER_EDGE, cbf)
pi.set_watchdog(G, 30) # get alert if quiet for 30 milliseconds

try:
   while True:
      time.sleep(60)
except KeyboardInterrupt:
   print("\nTidying up")
   cb.cancel()

pi.stop()