# Calculating RPM?

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#### Njguy

##### Member
I know that a tachometer shows rpm, but I would like to calculate the rpm of a rotary shaft, and I'm not sure how to do it. What is the hardware that actually gathers the rpm data?

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#### Mike_2545

##### Super Moderator
A slot sensor and a disk with a hole in it are a good way to go

#### Njguy

##### Member
A slot sensor and a disk with a hole in it are a good way to go

Ok ill look into that. Thanks.

#### Mr RB

Buy a $25 optical tacho, you can get them just about anywhere now (like tool stores, retailers, even Ebay is full of them). Just stick a white or black sticker on the shaft and point it, the tacho tells you RPM. #### Bob Scott ##### New Member Buy a$25 optical tacho, you can get them just about anywhere now (like tool stores, retailers, even Ebay is full of them).
Especially hobby stores, for counting model airplane propeller RPM.

Bob

#### Dean Huster

##### Well-Known Member
The Tektronix DC503 frequency counter had an internal slide switch that you could set to provide a display in RPM vs. Hz. Never did see any other counter that did that. I though it could be a really handy feature but never did use it myself.

#### unclejed613

##### Well-Known Member
Most Helpful Member
function generator in pulse mode, and an LED (high brightness white ones work best). frequency in Hz*60=rpm. as was mentioned above, a black mark or a sticker makes for a good "indicator" to use with the strobed LED

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