If you are in a plan to build your own speedometer for your next robotic vehicle, ofcourse you need an RPM sensor to complete the task. Today, almost all sensor modules for use with microcontrollers are readily available in pre-wired form. OK, but how about building your own? If you have the time and inclination, it may be cheaper and you could learn a lot of useful stuff in the process. Take an inexpensive Hall-Effect Sensor, couple it with popular Op-Amp IC and you have a rudimentary but workable Hall-Effect Sensor Module!
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.
The output null voltage (B = 0 G) is nominally one-half the supply voltage. A south magnetic pole, presented to the branded face of the Hall effect sensor will drive the output higher than the null voltage level. A north magnetic pole will drive the output below the null level. In operation,instantaneous and proportional output-voltage levels are dependent on magnetic flux density at the most sensitive area of the device.
In our RPM Sensor Module circuit, UGN3503 (H1) gives an output propotional to the magnetic field intensity, which is converted into logical pulses by the OpAmp LM358 (IC1). Sensitivity of IC1 can be adjusted using the 2K2 multi-turn preset potentiometer (P1). The 3mm Red LED (LED1) here works as a visual aid to indicate a highlevel (H) pulse output.
Gear Tooth Sensor is estimable for RPM sensing applications. For this, attach a small permanent bias magnet with suitable epoxy glue to the back surface of the epoxy package of UGN 3503. Next, fix your gear tooth sensor on the robot wheel (the presence of ferrous material at the face of the package acts as a flux concentrator). Note that, south pole of the magnet should attached to the back of UGN3503 if you want to sense the presence of ferrous material.
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.
Again, here note that applying a south-polarity (+B) magnetic field perpendicular to the labelled face (front) of UGN3503 increases its output, and applying a north-polarity(-B) magnetic field perpendicular to the labelled face (front) of UGN3503 decreases its output!