[jdmccright]

I can delve a little more into the details on the sensor and how it registers the signal from the "tone ring" that is on the axle.

As mentioned, the sensor utilizes the Hall effect to detect rotation of the axle. The sensor is basically an electromagnet (EM for short) that is fed a low voltage from the ABS or PCU modules. The result is a current signal that can vary between 4-20 mA (milliamperes). Left alone and away from other metallic items, the EM will generate a constant current. This current (any current actually) induces a tiny magnetic field around the wire. The sensor is a wire wrapped into many loops to amplify this field.

Now the Hall effect occurs because the magnetic field can be altered by other magnetic objects that come near the sensor's magnetic field and alters it...say the big tooth of a tone ring. Any change in the magnetic field also causes a change in the current running through that wire. So, as the tooth passes through the sensor's field, it causes a momentary change in current through that circuit. As the tone rings spins, the sensor will detect a pulsing current equal to the number of teeth on the tone ring for each full rotation of the axle. Do some math and you can determine the distance the tire rolls for each axle rotation.

You can certainly try to build one of these but if you're looking for something easy to rig together, I'd find a small speedo/odo unit for a bicycle and adapt that rather than building a whole electronic circuit from scratch. Around here they're around US$20-30 and would be good enough for tracking mileage. I do not know if your gov't or other business regulating bodies will accept this if they require trip meters.

Good luck!