Measure. - Here is Circuit Schematic RPM Sensor Module For Robotics w/ UGN3503. At the heart of this circuit is a Ratiometric Hall-Effect Sensor UGN3503. A Ratiometric Hall-Effect Sensor outputs an analog voltage proportional to the magnetic field intensity. It is one unipolar device operating at 5-volts. With no magnetic field applied the output is about one-half the supply voltage. The voltage will increase with the south magnetic pole on the face or decrease with the north magnetic pole on the face. UGN3503 includes a Hall sensing element, linear amplifier, and emitter-follower output stage. Problems associated with handling tiny analog signals are minimized by having the Hall cell and amplifier on a single chip.
If north pole of the magnet is attached to the back of UGN3503, absence of ferrous material is sensed by the setup. Needless to say, you can also try a Notch Sensor in lieu of the Gear Tooth Sensor. As a cheap alternative, you can try another simple method for rpm sensing. Fix the UGN3503 on the robot chassis, and fix a permanent magnet on the hub of the robot wheel so that a pulse is produced by IC1 whenever the magnet comes in line with UGN3503 as the wheel rotates. Use short-length of 3-wire cable for interconnection between the module and UGN3503.
The original article : "RPM Sensor Module For Robotics w/ UGN3503"