I found this script excellent. Thanks waaren!
@ayamy I had the trigger type as HTTP request, then it worked fine.

Think you can solve it using a combination of dummyswitch and user variable:

IF switch = On AND power < 250 AND uservariable = 0
{
dummy = On for 1 min
uservariable =1
}
ELSE IF switch = On AND power < 250 AND uservariable = 1 AND dummy = Off
{
Send your alarm message
uservariable = 0
}

Has anyone got this working with latest Raspbian Stretch?
On "step 10. sudo stty -F /dev/ttyAMA0 9600"
I get this:
stty: /dev/ttyAMA0: No such file or directory

Am I missing something? Something changed between Jessie and Stretch in regards to this?

[Update]
Solved it. had disabled the Serial port ...