I am installing a dust collector for my miter saw, and for the heck of it, I did a search to see how cheap current sensing switches have gotten. Well, it turns out, they have gotten cheap!
There are several out there- but here is one:
https://blackhawksupply.com/products...fDsaAlrQ8P8HAQ
You run one leg of the power to your saw through the hole, and you get a normally open set of contacts that close when current is flowing. Use those contacts to switch power to a relay that controls your dust collector, and bada-bing, you have an automatic dust collector!
Google "current sensing switch" and you'll find a lot of relatively cheap products. Yes, you can get down to the circuit level and build your own, but for an industrial quality control for $15, plus a relay (I have several laying around), I'll go that route.
FYI