My brushed dc motor spec. sheet lists BACK EMF CONSTANT (Ke) V/K R.P.M. 20.8. So knowing/controlling the voltage sent to the motor would make it easy to calculate, and set the rpm. My problem is my power source output is a constant 48V and my motor controller is PWM type so, when turning the pot, the voltage measurement also remains a constant 48V. Is there a digital display that can display actual rpms, voltage or some other parameter that would be easily converted to rpm?

  • \$\begingroup\$ A machine that graphs voltage is called an oscilloscope. Everyone who works with electronics should have one because you are essentially blind without it, but they are expensive. They're cheaper in recent years though but if you are a student you need to have a job and live rent free to be able to afford one. \$\endgroup\$
    – DKNguyen
    Commented Jun 13, 2021 at 22:02
  • \$\begingroup\$ There’s a hundred ways I can think that RPM can be measured by audio, electrical, mechanical, magnetic. Since your profile and question don’t hint your level skill or tools, what do you not want and what tools , skill set do you have? Budget? \$\endgroup\$ Commented Jun 14, 2021 at 0:21
  • \$\begingroup\$ Neiko 20713A optical tachometer. Great for measuring RPM on anything that rotates, as long as there is an accessible area. You may need to put a piece of reflective tape on the rotating object. US$ 30 on Amazon.com. \$\endgroup\$
    – user57037
    Commented Jun 14, 2021 at 2:17
  • \$\begingroup\$ You can also convert PWM to DC using a low-pass RC filter. So if you have a few resistors and capacitors lying around, and a volt meter, that may work. \$\endgroup\$
    – user57037
    Commented Jun 14, 2021 at 2:19
  • \$\begingroup\$ Why do you want to 'set the rpm'? \$\endgroup\$ Commented Jun 14, 2021 at 3:52

1 Answer 1


Maybe you can use a technique like this one to measure the RPM of a fidget spinner will a smart phone and a magnetometer app:


Some tachometers for fans work by shining a light or laser beam through the fan blades and measuring the frequency of interruptions using a light sensor. You can use this technique if your motor's load has something like blades or something that will block a light beam once or more times a revolution. You can use the light sensor on your smart phone and an app like phyphox mentioned in the video to measure the interruptions.


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