Serene Innovations Centralalert Notification System Ca360h Vibrating Alarm Clock/receiver with Hanging Door Knock Sensor for Deaf or Hearing Loss Impaired Disabled CA360 Vibrating Clock Receiver Features: An unlimited number of CentralAlert CA360 system can be used in close proximity, such as in apartment buildings or hotels Alerts to alarm clock, doorbell, door knocks, phone/fax, audio alarm, baby, weather, motion and more (sold separately) Supports up to 4 CA-DB doorbells and CA-DX door knock sensors (in any combination) Supports up to 10 CA-RX remote receivers, 10 CA-PX personal pagers, 10 CA-AX audio sensors, 4 CA-SOS emergency SOS pendants, 3 CA-BX baby cry sensors, 1 CA-CX phone sensor and 1 CA-NOAA weather alert sensor Alarm clock has adjustable brightness display, dual alarm settings and dual bed shaker jacks (one bed shaker included) 1.25" bright clock display Adjustable loud audio sound and bright flasher Big and easy to see alerting icons 2.4GHz frequency Uses 520Hz square-wave technology for maximum audio alert, found to be more effective in alerting people with hearing loss Up to 200 ft operating range Dimensions: 6.5" x 4.5" x 2.25" Battery backup 4 AA NiMH (batteries not included) CA-DXH Door Knock Sensor Features: 2.4GHz operating frequency Up to 210 ft operating range Uses 2 AA alkaline batteries (not included) Dimensions: 2.5" x 3.8" x 1" * Accessories sold separately.