Hall Effect Sensor.

[Caleb Fulton]

Here are a few tips on the Hall Effect sensors:

These guys simply output a digital signal corresponding to whether or not a gear tooth is in front of them or not, and the polarity of the signal depends on the direction of gear rotation wrt the sensor itself (see the data sheet for more details: http://www.allegromicro.com/datafile/0660.pdf). Think of this device as a one-bit wheel encoder when you program for it.

The air gap, according to the manufacturer, should be 0.5-2.5mm, though they assure us that their adaptive sensing technology can make it work for an even bigger gap, especially with larger gear teeth.

The device has an open-drain output stage, but the digital inputs on the RC have pull-up resistors built-in, so you can directly connect the signal to the RC's inputs. Try to avoid grounding the signal wire, though, as this would technically be a short-circuit. Don't worry if you do -- the manufacturer says it has short-circuit protection on the output stage.