We have one of the Chamberlain alarms and they do work well. We have a couple of receivers, and each has its own alert. Front and rear of the house are discreetly covered.
I change the batteries twice a year, sometimes three, if its a real cold winter.
Even with the alarms, the dogs always seem to know someone is here before they go off. They dont need batteries.
“The enemy is anybody who's going to get you killed, no matter which side he is on.” ― Joseph Heller, Catch-22
Beware of associating with "Cheap" Squirrels. They tend to be very narrow minded, hypocritical, and will gag you if you question or disagree.