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How to make RPM counters

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How to make an RPM counter?
Assuming you have some sensing mechanism,say a flywheel with a slit to pass the light beam that is sensed by a photosensor at reciever end and which is attached to a transistor driver  that gives you square pulses for every cut.
How to process the rest of the signal using a microcontroller?
One way is to use interrrupt pin to interrupt the MCU every time the pulse comes
and increment a variable in the ISR.
use a timer to generate a delay of 1 min.read the counter variable before starting the timer and after 1 min.take the difference and display.
Is there any more better way?
How are commercial RPM counters/taechometers made?
I feel they don;t wait for 1 min to gather the # of revs in a minute.
How do they work?

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Use a divide by N counter....say like CD4040 to divide yr input signal and then take to microcontroller  OR..
To avoid microcontroller ,  use CD4040 counter as divider , feed the divided output to Frequency to voltage convertor chip like NE565 PLL  and display it using 3 1/2 digit ADC for display , the type used in DVM (ICL7137)......This division approach will allow you to feed input signal to very high RPM and still not jam yr circuit........I hope things are clear.........feel free to drop by in case of questions....

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i think magnetic sensors work efficiently at high speeds compared to IR sensors.
u can uses a pair of magnet and halleffect sensor for calculating RPM.

i dono much abt pic programming.but i came across such RPM counter while searchig for LED POV stuff

scroll down,and at v2.2 tag u'll find a rpm counting led pov.

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The best way for a RPM counter is What pebe  Said.

Imagine if you use a Devider or revs in say 10 sec and then calculate for 1 min this problem will happen!

You should multiply the number of RPM in 10 seconds by 6 to achieve for 1 min. so the Number you achieve is alway = 6K. I mean : 6 - 12 -18 - 24- ... 6K (Sorry, I don't know the english name)

So if you have a real RPM = 15 then your device will show 12 or 18
And another problem is your counter will be refereshed every 10 seconds. If you decrease this interval (10s) then you should multiply it with a bigger number and ..... :o  :o

HTH - Shahriar

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