I want to measure RPM of a motor. For this, I am planning to put an assembly where an IR LED facing with IR sensor under line of sight. With every rotation, the line will be cut once - and hence the IR sensor LED.

Where I am stuck at is that once I get pulse of the signal, how do I count these pulses to make the Frequency (i.e. rpm) estimate? Is there a simple way to do it? I can use Oscilloscope - but I don't have easy to access to one. I have seen that most entry level multimeters don't have frequency measurement.

Also, unlike this question, I would also refrain from involving microprocessor based counting on this. [Basically this is a simple hobby project for high school student]

So is there a good and easy way to do it? Is there any completely alternative way to do this?

PS: The motor speed will typically be between 0-500 rpm not beyond. EDIT2: I want this to use a tool for physics experiments - so I need it as a quick indicator of rpm.

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    \$\begingroup\$ feed the output signal into the microphone input of your computer and use Audacity to record the signal at a known rate ... then examine the waveform and make measurements \$\endgroup\$
    – jsotola
    Mar 8 '18 at 8:50
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    \$\begingroup\$ you could also put a propeller on the motor and use an musical instrument tuning app on your phone to listen to the hum of the blade \$\endgroup\$
    – jsotola
    Mar 8 '18 at 8:52
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    \$\begingroup\$ The simplest way might be to use a frequency-to-voltage converter - LM331 or LM2917 are typical - with an analogue meter to measure the voltage. \$\endgroup\$
    – henros
    Mar 8 '18 at 9:30
  • \$\begingroup\$ @henros - that is something close to what i was looking at. Why don't you put this as an answer. Also, let me know if I should take any specific things in mind related to IR emit and reception to generate pulses. \$\endgroup\$ Mar 8 '18 at 9:36
  • \$\begingroup\$ I'll point out that microcontrollers are certainly within reach of a hobby project for a high school student, especially if you have some programming knowledge already. A lot of people start with an Arduino; it's a $20 board with a microcontroller that is designed to be easy to get started with, with a simple and user friendly programming environment. \$\endgroup\$
    – user253751
    Mar 8 '18 at 10:35

It seems like you want a tachometer.

here's one for $5:


seeing as you only want to go to 500RPM, To get more precision you can use a wheel with 10 slots (or 10 pulses per revolution) and divide the reading by 10.

It comes with an optical sensor, but that seems to be optional.


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