When I set to replace all our smoke detectors, I initially liked the SM120X relay, too. But wiring it up and then using a contact sensor seemed more of a jury rigged solution to me. Just installing the Ecolink a few inches from a hardwired smoke detector made it a wholly independent, simple solution.
FWIW, the audio tones for smoke and CO are well defined by an industry standard, so I don't think there is potential error there. The Ecolink it picked up the sound immediately and Hubitat processed the event right away. With a 5 year battery life (and using Device Activity Check to monitor it) I'm not at all concerned about the battery.
