Wireless Driveway Alarm puts an end to surprise visits, unannounced guests, and unwelcome salespeople by transmitting a signal each time anyone approaches your home. Simply position the weather-resistant transmitter outdoors, up to 400 feet from your house. Keep the receiver unit indoors so you can hear the alert tone. Uses one 9V and 3 AA batteries (not incl.). Each unit is 5" x 3-1/4".