Datalogging RPM signal

[Tom Sharpe]

Could you email me a sample circuit drawing? I am EE limited.  TNX Tom.

Phil Hunter wrote:

> "Tomas" wrote:
>
> > Hi, I'm writing an opensource program to plot HP
> > curves and monitor/log engine RPMs. I'm using a
> > voltage divider to pick the 12V signal from the
> > coil input. The input is recorded with my soundcard
> > using the Line input. I'm having troubles with
> > trying to figure out how to count the spikes,
> > because the signal I get is VERY far from a square
> > wave :). And with my ignition, when the RPM goes up,
> > the signal amplitude goes DOWN ....so no static
> > treshold for counting spikes :/.
>
> One suggestion would be to substitute a signal diode
> such as a 1N916 or 1N4148 for the "bottom" resistor in
> the divider, cathode (band) end to ground. This should
> give you a relatively constant 0.7V high "pulse"
> waveform across the RPM range. If your input signal
> goes below ground, then add another diode in parallel
> w/ the first but connected "backwards" (anode to
> ground).
>
> If you truely want a square wave, you may have to run
> the signal above into an op-amp to amplify it again,
> then into the clock input of a flip-flop IC to make a
> square wave at half the input frequency, then into
> another voltage divider to bring it back down to a
> level compatible w/ the line input. I'd probably use
> CMOS for good noise immunity, low power, and wide
> power supply tolerance, just don't let the input
> signals exceed the power supplys (Vcc or ground) or
> you could destroy the chip. Also don't let any unused
> inputs "float", tie them to Vcc or ground. If this is
> a laptop, you can "steal" power (5V) from the external
> keyboard connector, in my experience it tends to be
> electrically noisy (I used it in place of batteries
> for some external speakers and you could hear the
> noise) so you'd probably need some filtering.
>
> [snip]
>
> regards,
> philh
> (digest)
>
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