power_monitor.py

#!/usr/bin/python
import os
import syslog
import time
import RPi.GPIO as GPIO

# Number of minutes to wait before shutting down
SHUTDOWN_GRACE = 2
# Pin connected to power detect line
POWER_PIN = 17

GPIO.setmode(GPIO.BCM)
GPIO.setup(POWER_PIN,GPIO.IN)

try:
	POWER_AT_START = GPIO.input(POWER_PIN)
	if not POWER_AT_START:
		syslog.syslog("Power detection line isn't connected. Will monitor for connection.")
		GPIO.wait_for_edge(POWER_PIN, GPIO.RISING)
		POWER_AT_START = True
	syslog.syslog("Power status line detected. Monitoring for power interruptions....")
	while POWER_AT_START:
		GPIO.wait_for_edge(POWER_PIN, GPIO.FALLING)
		# The next 2 lines weed out false positives and chatter.
		time.sleep(5)
		if not GPIO.input(POWER_PIN):
			out = "Power disconnected. Waiting "+str(SHUTDOWN_GRACE)+" minutes until shutting down."
			syslog.syslog(syslog.LOG_ALERT, out)
			os.system("echo \""+out+"\" | wall")
			time_power_lost = time.time()
			while not GPIO.input(POWER_PIN):
				if time.time() > time_power_lost+SHUTDOWN_GRACE*60:
					os.system("shutdown -h now")
					time.sleep(60)
				time.sleep(1)
			time_left = time_power_lost+SHUTDOWN_GRACE*60-time.time()
			out = "Power connected. Aborting countdown with "+str(time_left)+" seconds to spare."
			syslog.syslog(syslog.LOG_ALERT, out)
			os.system("echo \""+out+"\" | wall")
	syslog.syslog("Power detection line isn't connected. Aborting power monitor.")
finally:
        GPIO.cleanup()