In my next project it will be very helpful to monitor the speed of my dc motor. How can I accomplish this?


2 Answers 2


In addition to the cornucopia of methods that burst forth from Russell's answer you could consider to add yet another to that rich bounty- if you have sufficient off time in your maximum PWM duty cycle you may be able to harness the back EMF of the motor itself as a tachometer signal. This has the advantage of being sensorless. It can be a suitable method for motors that are permanent magnet or that have a separate field winding. There must be sufficient time for the energy stored in the rotor inductance to die down, after which you can simply sample the voltage with an ADC.

A detailed description of this method can be found here. The back EMF is proportional to the RPM of the motor, and of the same polarity as the applied voltage (assuming a PM type or fixed field voltage) when measured at the terminals.

  • 1
    \$\begingroup\$ This looks to be a little above my head but I love that it's an "out of the box" approach. I'll definitely give this a shot. \$\endgroup\$
    – Jacksonkr
    Feb 8, 2015 at 20:14

There are so many possible ways that the best way depends on the details.
Some suggestions

Pulse per revolution from mechanical or magnetic or optical switch or ... .
Triggers could be "cam or bump", magnet, slot or mark or light or ... .

Or several pulses per revolution using either multiple sensors or multiple triggers.
Extending to ...

Tachometer track - series of marks or slots or magnetically variable permeability disk or ...

Small alternator/generator attached to shaft (often known as a tachometer).

Once you have one or multiple pulses per revolution you can

  • count the number over a given period, or

  • measure the time between them, or

  • trigger a fixed length pulse per input pulse (known as a retriggerable monostable) so that the mean DC level out is a measure of speed.

  • or ....

Where the output is a period you need to take the reciprocal to get speed. A basic microcontroller such as an Arduino can do this easily. THese can be obtained for as little as $US2.85 each if you buy 10 or $US4.50 each if you buy one.
Get a few friends to buy one too and get 10! [No, I have no relationship with them except as a happy customer]


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