Brian Gaff explained on 18/04/2019 :
Tee hee.
No the magnet and hall effect sensor work well, but it would be much cleverer
to somehow link the on off pulses so that if they were not present it cut
the power.
Brian

That would involve much more complexity than it needs. It would require
some means to override it, to enable the engine to be started.

My first attempt at an indicator was a simple hinged flap on the output
of the duct. Air blown out by the cutter fan, lifted the flap, but it
was impossible to see the flap whilst driving. No air flow, meant the
deck was choked. I suppose I could fit a magnet on a flap, operating a
reed switch to turn on an LED.