Check the listing for details. Serene Innovations CentralAlert CA-AX Audio Alarm Sensor. Condition: New. Listed at 45.95 USD. The Audio Alarm Sensor is part of the CentralAlert Notification System by Serene Innovations. It sends to the CentralAlert receiver to notify you when audio devices such as smoke/fire or carbon monoxide alarms are activated. It also works with newer T3/T4 alarms. It can be installed next to the audio sources or directly connected. Features:Mount next to audio alarms for alarm detection or hardwired to alarms that have connecting terminals for direct activationSends a wireless signal to the CA360 receiver2.4GHz Smart-Code technologyUp to 200 ft operating rangeSupports up to 10 CA-AX audio alarm sensorsLow battery indicatorUses 2 AA alkaline batteries (not included)Dimensions: 2.5" x 3.8" x 1" What else do I need to make this work?In order to be notified, you will need to purchase a compatible receiver (see Accessories box).