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Arduino Tachometer

Posted: Tue Oct 29, 2013 9:36 am
by mohd.ahli
Hi Chris
I tried your project but i seem to be having funny readings about the RPM on my lcd... sometime i get very high values and sometimes i get a negative rpm value.

The only thing i changed on the circuit is that i used 2 2n3904 NPN transistors and the resistor for T3 i used was 24kilo ohms.....
Would like to know whats the problem.

Re: Arduino Tachometer

Posted: Fri Feb 20, 2015 5:41 pm
by frank00
Hello Admin,

I would be very grateful to you if you could reply to the above post. I am having the same problem.
RPM values are very erratic, sometimes large negative values are obtained.
RPM value never becomes stable and does not return to zero when rotor blades are removed.

I made the circuit exactly as in your schematic.

Thanks in advance,


Re: Arduino Tachometer

Posted: Fri Feb 20, 2015 11:54 pm
by ThePyroElectro
The design I made is not flexible or very redundant. Because of that if your fan's speed varies much in RPM from mine it's possible that erroneous data will be displayed.

The best thing to do is use an oscilloscope on the interrupt pin (digital pin 2) to make sure the interrupt is indeed occurring when expected and that it is a falling edge. Large values could occur if falling edges are rarely detected. Negative values should never be able to occur, I simply cannot explain why that would happen.