Richard wrote,
> My question is: Has anyone taken the stock tach and
>removed all of the guts leaving (basically) the shell and the face
>plate, and then added an aftermarket tach with basically the same RPM
>sweep range, an no one is the wiser as far as operations goes?
>
>Rich
The answer is yes, and it is really simple too.
I used a microprocessor for extra accuracy, but that is overkill.
The same could be accomplished with the following parts.
7805 5v regulator.
74HC123 retriggerable multivibrator IC.
Polypopylene capacitor "for temp stability and accuracy"
a couple of resitors and a scaling potentiometer and another
lower grade "ceramic or mylar" cap.
total parts cost under $5.
The major contributor to accuracy here is the variation in the
output voltage of the 7805 as a function of temperature.
This can be elliminated with some clever design though.
The multivibartor puts out a fixed time pulse into a low pass filter.
as the revs increase, the amount of time the pulse is high relative
to the pulse being low decreases.
What you get is the a traditional "Tach" circuit, that does not
suffer the normal maladies that our Smiths tack have.
Jarrid Gross
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