Using Flow Control to Switch Bathroom Fan

I'm just having a choice about extractor fan for a bathroom.
It seems that you can have a simple switched unit which then requires
manual operation and a second pull-cord, Timed from the light pull-cord (which is irritating if the fan goes off for a nocturnal trip to the loo and won't usually be activated during the day) Or on a humidity switch, which seems very expensive.
As the shower pump has a pair of flow sensors on it to switch it on and off, would it be electicially possible to use that to trigger a timed extractor fan - ie, using the flow sensors instead of the bathroom lightswitch?
I can see it working in principal, but don't know if there'd be problems with the fan drawing too much current or their being some sort of feedback which might prevent the shower pump from switching off ...
Any thoughts?
Thank, James
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