[LAU] A crazy idea

[Gabriel M. Beddingfield]

On Thu, 4 Mar 2010, Karl Hammar wrote:

> Why don't you put an inductive sensor [1] at the pedal gear teeth?
> You could probably mount it at the chain protective cover if the
> tolerances are the right.  Have a look at Omron "E2E-X5MF1 2M" [2,3]
> (you might have to paint it to protect it from water). For that one
> you need 12-24V DC.

This is a good idea... as long as your sensor and your uC 
can keep up with the pulses.  I didn't look into the sensors 
that you picked, but they usually publish their response 
times... so you need to check that out.

How many teeth does a typ. sprocket have?  Say about 40T. 
Lance Armstrong spins his cranks at 80RPM.  That would be 
53.3 pulses/sec (or 1 pulse every 18.75 ms).

Sometimes the sprockets have spokes or holes in them to make 
them lighter.  Like 6 or 8.  Perhaps sensing those would 
provide a more sensible signal if 18ms is too fast for your 
sensor/uC.

-gabriel