The Gentex GN503 is a single or multiple station combination smoke and carbon monoxide alarm. It's designed for 120V hardwired use and features a 9V battery backup for ensured safety even when power is lost. This Gentex smoke/carbon monoxide alarm features a temporal 3 (T3) audible signal for smoke detection and a temporal 4 (T4) audible signal for carbon monoxide evacuation. This model features a universal mount that can be use for ceiling or wall applications.