2 Beams wireless AIR barrier. Range: 30m Works with D size lithium battery from 3.6 to 3.9VDC. (SB-D02HP from VITZROCELL recommended) (2 for each transmitter / receiver) or CR123A 3.0 VDC lithium battery (8 for each transmitter / receiver). Easy frontal access to change batteries (Batteries not included).