Datalogging RPM signal

[Phil Hunter]

"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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