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My Ender 3 enclosure has had a smoke detector installed pretty much since I built it. I recently got around to upgrading it a bit so that printer power would automatically be killed if the detector went off. This was done by using a detector that is built to be linked to other detectors plus a relay designed for use with that style of detector.
The smoke detector was $12.25 at Amazon (prime) First Alert Smoke Detector Alarm | Hardwired with Backup Battery, BRK9120b6CP
The relay was $16.89 at Amazon (prime) BRK RM4 Smart Relay for First Alert
The specs on the relay looked to be plenty adequate for the Ender 3 power draw (15 amp resistive or 1/3 HP motor). I wired the relay into a short (2') extension cord so the printer power gets interrupted if the smoke alarm goes off.
I used some round 3 conductor antenna rotor cable for the connection from cord to smoke detector pigtail. All connections were soldered & covered with multiple layers heat shrink. I didn't bother with an enclosure for the relay assembly, it was already heat shrink sealed as a bundle, so I just taped the connections together to make a neat packet.
Wiring was pretty straight forward. The pigtail for the smoke detector wanted hot & neutral wires (from the plug) plus a third for the signal. The input for the relay needed signal (from the detector) + hot & neutral (from the plug). On the output side, the relay common got a connection to hot from the plug. The socket end of the cord got ground & neutral from the plug end + the relay's NC wire for the hot side.
Tested it out before plugging the printer in (had already checked wiring cold for continuity & shorts). It dropped power on the detector test button being pressed & restored power about 5 seconds after the beeping stopped. This assembly got plugged into the power strip from the UPS I use for the printer.
It doesn't address everything that could go wrong, but if I need to leave the area for a bit it makes for a bit of insurance while I'm out of earshot.