Idea about RMP calculation

[thebadtall]

Hello I would like to calculate the RPM's of my old vespino.
Since it is difficult to screw an encoder in the stator, I thought of
measuring the ac voltage (of generator) through a 10k voltage divider and measuring the peak voltage phase time , or using the pulseIn command with arduino.
Or using interrupt Rising or Falling.

Is that a good approach ? Should I use another approach, ie measuring the phase time from an optoisolator ?

Thank you

 

[wp100]

Hi,

Have you tried using some insulated wire coiled around the ht lead or a connection the low tension contact breaker if it uses one ?

 

[thebadtall]

No, I havent yet started. I am thinking if voltage measuring is a good approach

 

[PA3040]

You can have pulse from ignition coil input

 

[wp100]

No, I havent yet started. I am thinking if voltage measuring is a good approach[/QUOTE

Hi,

Would have thought it has some form of voltage limiter fitted to avoid blowing things if it was over revved ?
Similarly if it cannot push out a good voltage at lower revs you are going to have a flat battery / poor lights etc.

 

[thebadtall]

Yes this is a nice approach too. Do you mean to read the coil from stator wich feeds the ignition coil ?

However, there is no regulator or capacitor or anything in the alternator wiring as it is the old 6volt vespa

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Would have thought it has some form of voltage limiter fitted to avoid blowing things if it was over revved ?
Similarly if it cannot push out a good voltage at lower revs you are going to have a flat battery / poor lights etc.

Hello, it hasn't protection, no battery or limiter. Indeed at low rpm lights blink

 

[BradtheRad]

Here is a simple frequency-to-voltage converter, made from a high-pass filter, followed by a peak detector.



By careful adjusting of the rolloff curve, you can obtain a response which is reasonably linear.

Because of the diode, the meter may show a zero reading at idle rpm. This might be cured by making the peak detector from a transistor.