Interlogix TX-E221 Recessed Door/Window Sensor Recessed Door/Window Sensor, 319.5MHz. The TX-E221 Recessed Door/Window Sensor from Interlogix is compact and robust for dependable operation. Hidden within the door frame, this contact is virtually invisible when the door is closed making it ideal when discreet sensors are needed. The TX-E221 Recessed Door/Window Sensor uses a replaceable lithium battery that lasts up to five years under normal usage and comes with a standard flanged top. Field proven wireless technology Compact design Easy replaceable battery Wide gap up to 1" maximum Replaceable battery with up to 5 years of battery life(based on usage)