Measuring RPMs with a Digital Multimeter

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SUMMARY

This discussion focuses on measuring RPMs of a DC motor using a Digital Multimeter (DMM) that can measure frequency. The proposed method involves attaching a flywheel with a magnet to the motor shaft, which would trigger a Hall-effect sensor to relay data to the DMM. An alternative method discussed is using a flywheel with a slot and a light source paired with a photoresistor. The consensus is that the photo-sensor method may provide less noise, and the concept of an encoder is highlighted as a relevant keyword for further research.

PREREQUISITES
  • Understanding of Digital Multimeters (DMM) and their frequency measurement capabilities
  • Knowledge of Hall-effect sensors and their applications
  • Familiarity with photoresistor functionality and light-based sensing
  • Basic principles of rotary encoders and their types, specifically vane encoders
NEXT STEPS
  • Research the design and implementation of rotary encoders
  • Learn about Hall-effect sensor applications in motor speed measurement
  • Explore the construction and use of photoresistor-based RPM measurement systems
  • Investigate the specifications and features of various Digital Multimeters for frequency measurement
USEFUL FOR

Electronics enthusiasts, hobbyists building motor control systems, and engineers interested in RPM measurement techniques for DC motors.

LittleBrother
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I have a Digital Multimeter (DMM) which also measures frequency. Can this be used somehow to show RPM's of a DC motor while it is running at various stable speeds? Or is there an inexpensive device which can be used to show RPM's?

If I have to build a device, my thoughts are to mount a sort of flywheel on the shaft of the motor with a magnet attached to the flywheel. The magnet would pass by a Hall-effect sensor. Then I could feed the output from the sensor to the DMM. Would this work?

An alternative would be to use a flywheel with a slot cut into it, along with a light source aimed through the slot, and a photoresistor on the other side, then feed the output to the DMM.

Thoughts anyone?
 
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I think your alternative idea would be the best. I've got a feeling that the photo-sensors would be much less noisy than a Hall probe.
 
You are making what is called an encoder (consider that a key word for a web search) A common low precision form of encoder, a vane encoder, is similar to what you describe, frequently the wheel will have multiple slots.
 

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