A Better Pedal-Assist Sensor?

[byebyepetrol]

On my front-hub electric assist bike the ring of magnets / hall-effect sensor system on the BB seems popular but is subject to grit and grime and suffers as a result.
The neodymium magnets pick up metal particles and clog up. They need cleaning regularly.

I can't help thinking that a better solution is to use optical detection of rotation.

Is there an optical trigger system on the market that is a direct swap-in replacement?
If not, is anyone here on ES savvy enough to design one?

 

[d8veh]

There is one. I have a couple of them. They were fitted on some Chinese bikes a few years ago. The sensor is about 15mm thick, which requires a much longer non-standard BB spindle, which gave other problems - worse than what you've described. hence they seemto have given up with them.