The Mighty Mule FM231 Wireless Driveway Alarm Instantly alerts you when someone pulls into or out of your driveway. The outdoor electromagnetic sensor only detects vehicles; no false alarms due to animals, children, etc. When a vehicle passes by the sensor, an outdoor transmitter communicates wirelessly to an internal base station up to 400 ft. away and sounds a pleasant chime. Easy-to-install Electromagnetic Sensor Wand is buried next to your driveway, and detects mass metal moving within a 3-12 ft. radius of the wand. Ideal for home and business. Requires 2 AA batteries (not included). Low battery indicator on both the Indoor receiver and transmitter.