Larry Jaques fired this volley in
:

LJ's 2-cents: Optical seems the best route. If it's a dirty enviro,
shield it, wot?


Still... why not a vibrational pickup? Then there's _nothing_ to get
dirty, no imbalancing of the engine, and no 'attachments'. Just mount the
pickup _near_ the motor mounts and be done with it.

They've got them in teesy sizes that will handle up to a couple of hundred
KHz, can consume NO power (some active ones will pull a couple of mA.)

Lloyd