SRL 400 wireless bidirectional bell battery included. is a two-way supervised wireless bell in triple-frequency (433.27 MHz, 433.92MHz, 434.57 MHz) collision. auto-learning programming, internal antenna with high efficiency;internal battery back-up 7.2V 700mAh Ni -MH batteries. Low consumption wide range of external power supply with (10V to 15V, 150mA max) aHigh efficiency LED light.