DickN (dickn@hpspdbc.spd.hp.com) writes:
> Andrew, I suspect your description of this unit leaves out a step. What you
> have described seems to be a guage which reads ON-time vs OFF-time rather
> then rate. Your description seems to fit a DWELL meter. It you add a stage
> containing something like a 555 timer chip which would trigger on the point
> pulses and be ON FOR A CONSTANT LENGTH OF TIME independent of the length of
> time the points are on it would work.
You obviously know more about electronics than I do (MOST people do :-), and
I was trying to avoid going out too far on a limb because I knew someone would
come along and chop it off. Unfortunately I can't open up the tach easily due
to GM's inspired assembly design of 1970, but I'm assuming that it's a really
simple scheme they use because (1) the whole thing works off of one single
wire supplying the rapidly-switched 12V, and (2) hey, we're talking GM here.
I don't think it's comparing On time vs. Off time because that ratio would
remain the same regardless of engine speed, but the rest of your description
fits: it must be making some sort of crude calculation on the FREQUENCY of
the incoming on/off cycles that results in a varying (voltage?) value it can
display on the indicator needle.
Sorry to bore the net with this, but I thought some LBCs out there might be
using similar tachs...
Andrew C. Green (312) 266-4431
Datalogics, Inc. Internet: acg@dlogics.com
441 W. Huron UUCP: ..!uunet!dlogics!acg
Chicago, IL 60610 FAX: (312) 266-4473
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