Seems to me that the main alarm bit and raising help is the easy bit. It's how to detect that you are OK and without generating false alarms or needing you to constantly reset things - now that's the tricky bit.
Mmm...get some motion sensors...ideally low-voltage ones like those from Voltek that can be ganged together. Arrange to give coverage over the whole workshop. Typically for me, at any rate, it's a constant movement between SCMS, planer/thicknesser, table saw, spindle moulder, bench, wood stock etc. and so these will constantly be generating a signal.
Then have a countdown timer....which is reset back to the start of its countdown by this signal from the sensors created by your movement. If the timer reaches zero then it kicks off a second shorter timer with an audio alert (in case you have got so engrossed sharpening all those chisels at your workbench that you've been static for a long time). So you can reset everything before the main alarm kicks in if you are OK.
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