I have this gear (image attached) that I'm putting over a metal shaft and I want to measure its rpm from 800-15,000 rpm with a tolerance of +/-10 rpm.

enter image description here

I'm thinking I can do this using a hall effect sensor (the type with a magnet built in so you can measure non magnetic metals moving) and an Arduino. I need help on what parts I need (digital vs analog hall effect sensor) and if this is a good way to do this. Thanks.

  • \$\begingroup\$ Are you referring to a hall effect sensor or a reluctor? Reluctors are commonly used in automotive as sensors and usually have a built in magnet. \$\endgroup\$
    – Kartman
    Oct 27, 2021 at 1:58

1 Answer 1


A magnet-coil (variable reluctor) or Hall effect (semiconductor sensor with amplifier) can detect the passage of a steel tooth. The reluctor will give a pulse (for each tooth) of a constant number of volt-seconds, so the signal gets smaller at low RPM. A Hall sensor will give a pulse proportional to field strength, so the voltage is near constant (and if the amplifier has hysteresis, is of logic character).

Nonmagnetic metal will be problematic at low RPMs; steel will give a better signal, unless the teeth are large compared with skin depth of the material at the frequency of interest; the depicted 16-tooth item, at 800 RPM, will give about 200 Hz, so (for aluminum) greater than 6mm. You'd also want the magnet field reach to be greater than 6mm.

A counter/timer software can compute RPM from this, and if you use a digital sensor, it will not require any amplification to drive the Arduino I/O pin.

Mainly, there will be need to position the sensor and affix the gear to the shaft so that the signal has a margin sufficient to always trigger when a tooth passes; too close, and it could detect the entire shaft, and stay ON. Too far away, it could fail to detect a tooth.

Age and temperature and mechanical wear will alter the signal slightly, so a bit of position experimentation is called for; you might want to get a sensor in a machine-bolt form, so its position can be easily adjusted, in a fixture with a threaded bore. That's convenient, but expensive in off-the-shelf hardware. Optical lamp/sensor assemblies and striped or perforated targets are an alternative.


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