tachometer

[madhunm]

Hi all,

I am trying to build a tachometer or rather, if you look at this application, a frequency counter.

What I have in mind is this :

connect the HT lead going into the spark plug as an inductive supply to a 555 based monostable vibrator. everytime the HT lead conducts, the monostable is triggered and a square wave is output.

i want to connect this square wave output into a pic and measure the frequency. now the math to calculate frequency is pretty simple.

RPM = frequency * 60;

can someone help me measure this frequency?

in case i forgot to mention, the RPM is displayed on a LCD (obviously).

thanks in advance,

Madhu.

[prtsoftware]

We are doing the same thing, but have settled on using a LM2917 frequency to voltage chip. Our reasons for choosing this design was signal-cleanup and voltage normalization.

We don't quite have everything all hooked up, but the output signal looks nice on the scope and gives a voltage that is nice and linear across various speeds.

See http://www.national.com/ds/LM/LM2907.pdf

Also search for "tachometer" in the forums to see the recent discussion about measuring Speed on a Toyota...I think.

Tom Johnson

[madhunm]

Hi Tom,

i realised the potential of the 2917; however, on the datasheet, it says something about 67Hz/V (in the application diagrams).

i did not quite understand how it works.

thanks,

Madhu.

[prtsoftware]

Depending on how you wire up the chip (using preamps, varying the capacitor , varying the resistors) the system will be able to handle different frequencies.

I don't have the values in front of me, but we configured the chip to work with a hall sensor to measure the revs of a diesel motor. It took a bit of trial and error but we now have a good analog signal generated by it.

I believe that you can request chips from National Semiconductor.


Good luck,

Tom Johnson

[prtsoftware]

One more thing...we were perplexed by the 67Hz/volt notation too. Not sure what it means, but the chip should handle the average flywheel on the average engine.

Tom

[madhunm]

Hi Tom,

I finally figured out what the 67Hz/V means... I think...

What I think it means is this : it needs 67Hz to output 1V.

Anyways, do you think you can share the schematics of your Tacho? I am using this for a single cylinder 2 stroke petrol engine.

thanks.

Madhu.