Hall Effect Sensor.

[Sparks333]

Originally Posted by Al Skierkiewicz

Mike,
I can't get to the page you are listing.
So for the meantime, think of this. You are able to get two pulses from two seperate sensors. As the gears turn, one sensor has an output slightly different in time from the other. (You are positioning them that way for this demo.) Each outputs exactly the same info, (error checking) and if you determine which sensor outputs first for a particular direction then you will always know that you are traveling in that direction. When the pulses change places you have reversed direction.

Al,
I was looking over the allegra datasheets (links up above... Thanks Caleb Fulton!) and on Page 9, it describes something regarding polarity and direction. I think its internal circuity makes it so when you are facing a tooth going one direction, it's high, but when you are going the other direction, it's low. If your gear teeth are fairly far apart, you would have to measure the duty cycle to see which way it's turning... not sure how it would be done, but it may be possible with a single sensor to tell direction.
Here's what I read. Please correct me if I'm wrong.

Output Polarity: The output of the device will switch from off to on as the leading edge of the target passes the sensor in the direction indicated (look at the datasheet for the pic), which means output voltage will be low when the unit is facing a tooth. If rotation is in the opposite direction (again, see URL for datasheet), the output of the device will switch from on to off as the leading edge of the target passes the sensor, which means that the output voltage will be high when facing a tooth.

http://www.allegromicro.com/datafile/0660.pdf , pg.9