The fume extractor on this bench has one switch, and the failure mode of a manual switch is always the same: it gets left on all day, or forgotten entirely while flux fumes build up over an hour-long assembly session. Neither is a hardware problem worth a clever solution, except that a clever solution turned out to be about ninety minutes of work.
A reed switch, not a current sensor
The first instinct was to sense current through the soldering iron itself and trigger the extractor from that. It's the more 'elegant' answer and it was also solving the wrong problem: the iron draws current constantly while idling at temperature, not just while actually soldering, so a current sensor would just leave the extractor on permanently anyway.
The actual solution was a small magnet glued to the iron and a reed switch in the stand: extractor turns on the moment the iron is lifted out of its stand, and a simple RC-timed relay keeps it running for three minutes after the iron goes back in, long enough for residual fumes to clear.
Four channels on the board because the stand has four iron positions, and it was no more expensive to build all four than to build one. The extra three channels sat unused for a month before a second iron showed up on the bench, a case where over-building by a small margin cost nothing and paid off anyway.