Methods of measuring RPM's of spinning object?

[spinningmagnets]

I have seen laser-tachometers from Harbor Freight (a national chain of Chinese tools in N. America) that sell for $40, rated for 99,000-RPM's, shines a light and a reflective strip on the spinning object sends light pulses back to the optical sensor. Reviews online are spotty, some love it, say it works and is cheap...others say its crap.

I can tape a small magnet onto something (that is spinning roughly 360-RPM / 6 times a second), and feed 5V to a hall sensor that I hold up to the hub. How would I measure the magnetic pulses and capture that to read out in a digital format?

I'm also open to any other ideas anyone might have, optical sensors are cheap, too...

 

[23skidoo]

You may have an ordinary bicycle speedometer which can do this task well.

 

[spinningmagnets]

Thats worth a try, sounds easy and cheap.

 

[teklektik]

Bike computer is the way to go:

• Use a bike computer like the Cateye Velo 7 ($18 Amazon):
• Set the wheel circumference to 1666mm (the Velo 7 uses integer cm, so round up to 167cm (error = 4/1666 = 0.24%) )
• Set the units to kph
• Set the secondary display to "Max Speed"
• Read the RPM directly divided by 10 (20.4 kph = 204 rpm)

For instance:

• 

View attachment CC-VL820520_HP_ENG_v3-1.pdf

 

[John in CR]

Set the CA to Km and wheel to 1667mm. Then speed X 10 = rpm. If you're not going over 1000rpm, then the CA may accept 16667mm and report rpm, but I haven't tried that.

 

[spinningmagnets]

Thanks! Both of those suggestions are home runs! I have both a CA and also a cheap bike speedo (but I may buy the Cateye Velo 7 as a spare)

 

[Ykick]

There's also smartphone apps which use camera flash to variably strobe a label or something on your rotating part. When it appears stationary the strobe freq is converted to your RPM.