There's an article in Fine Woodworking - July/August 2000Pg 66 - titled
"Dust Detector" "Switch automatically turns on dust collector when machines
are running." I was reading it last night - could be just what you're
looking for. Gives wiring diagrams for 120 and 220 plus pictures and
graphics... Looks easy to follow and the total cost at that time was about
$60 for the parts, the mos6t expensive of which was a current sensor.

It gives sources for the parts as well (Granger for the TCSHAA sensor/relay)

Good luck - You're wiring in the shop sounds like what I'd like to have!

Keith Piercey