What Sensor to Measure Rotation?

Hi, looking for suggestions to meaure the rotation in a spinning speaker. There are two spinning elements I need to measure seperately. One is the bakelite horn, like this: http://users.sa.chariot.net.au/~dna/gif/leslie_slo.gif

There is also a rotating wooden drum on the bottom: http://en.wikipedia.org/wiki/File:Leslie_Speaker.jpg

Putting anything too heavy on the rotating parts will disrupt the balance, so I was thinking optical, like put a strip of whiteout on the horn and drum??? I suppose I could get away with glueing a small magnet or aluminum to the parts.

Hall sensor? Optical?

Thanks, Carl

DO you want to build the device inside the speaker or outside teh speaker?

1) digital compass 2) distance sensor or a rotary encoder ...

My first thought would be a reed switch: http://en.wikipedia.org/wiki/Reed_switch Small magnet on your speakers. One close of the switch per rotation. Simple and cheap.

It would be inside the speaker. I was thinking if I could glue some aluminum foil or small magnet to the horn would be the way to go. I would want the sensor mounted static to a shelf inside. Looks like I will try the reed switch. Thanks all!

What about optical? Similar to what this guy did to measure RPM. http://www.youtube.com/watch?v=0tMiqLMih34

I am looking to do similar thing, but to measure RPMs of a lathe to ensure I have accurate speed readings.

-- Jim

so I was thinking optical, like put a strip of whiteout on the horn and drum?

Yes that is exactly what I would go for.