At 03:20 PM 3/24/2011, Rocket J Squirrel wrote:
>Truth is, I have one of those.
>
>But it, my digital one, and the dash tach all give different readings.
>Was hoping to find an analog one that is known to be pretty accurate and
>with a 4-cyl setting. Analog because the digital one is always goofing
>around, waiting for the rpms to settle.

Be surprising if the digital were very far off.  But make a nice
divider that will give you maybe a milliamp max and 100v peak and a
cap to differentiate the sine wave and get spikes instead (and maybe
a diode so the spikes are all the same polarity?) and calibrate
it/them off the AC line.

NOTE:  Mr. Squirrel knows how to do this without getting himself
killed.  Most of y'all don't, which is why I'm not providing component values.

Yrs,
d