--- tinker man <tinkerman007@GMAIL.COM> wrote:

> Also, looking at the schematic, I see that the coil driver output goes
> in parallel to the dashbord to drive the tachometer,

Before taking this signal to your counter you will need some RC signal
conditioning circuit to shape/filter the signal for the counter. Without
signal conditional it may be too noisy for the counter.

When the engine idles you will only see about 30 pulses per second of this
signal. But you will like to see 900 (RPM) displayed on the counter. But
you will see 870, 900 or 930 if your counter is set to count the pulse
at a rate of once per second. If you want to see 898, 899, 900, 901, 902...
(higher resolution) you may need to set your counter to count once per
minute. This will be a poor RPM display because it updates only once per
minute.

The best way to design a digital RPM display is to measure the duration
of the pulse of the coil signal, instead of counting the pulses, which
has a limited resolution. By measuring the duration of the coil signal
you can make a fast updating precision RPM display. You can read 901.25
RPM at a refresh rate less than a second with a precision to +/- 0.01 RPM.
It is still a counter. But it counts an oscillator during the duration of
the coil signal. If the oscillator's frequency is high enough then you
get very high resolution.

David

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