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From: jeh@cmkrnl.uucp
Newsgroups: sci.electronics
Subject: Re: Digital dash (was: Inductive pickups.
Message-ID: <1991May30.151040.50@cmkrnl.uucp>
Date: 30 May 91 22:10:40 GMT
References: <1970@ole.UUCP>
Organization: Kernel Mode Consulting, San Diego CA
Lines: 47

In article <1970@ole.UUCP>, ssave@ole.UUCP (Shailendra Save) writes:
> [...]
>   Sometime ago, I asked the net about a weekend project to
> make a tachometer for my car. I got numerous replies, for
> which I am thankful. Hopefully I will soon have a complete
> digital dash. I will post a summary for those interested.

Projects for such have been appearing in the hobby mags every now and then for
almost forever.  There was even one in one of the trade magazines, _EDN_ I
believe, circa 1974 (!) or 75 or so. That one was for a motorcycle. 

Personally I would steer clear (ha ha) of a digital dash.  Saab, who builds
fighter planes as well as cars, has done extensive research on what kinds of
gauges can be read the most easily.  The answer?  Old-fashioned analog, with a
moving pointer against a stationary background.  The pointer and scale can be
electronically generated (ie on a computer-generated display) or mechanical,
circular or bar-graph style, but whatever the variant, it beats a digital
display six ways from zero.  You see, it is usually far more important to know
that "you're nearly at redline" than that "engine speed is 6,253.8 RPM", and
it's MUCH more important to know that "your oil pressure is low" than that
"your oil pressure is 21.3 psi". 

For rapidly-changing things like a tach, the pointer and scale have another
big advantage:  It is easy to see how fast the reading is changing.  This is
almost impossible to derive from a digital readout.  

Color-coding the scale for good, warning, and bad ranges -- or adding
color-coded idiot lights -- obviously helps to draw attention to those gauges
that need reading at the moment.  But color-coding gets tricky even for
something as simple as an automobile engine.  For instance, my water temp.
gauge swings almost up to the red part of the scale as the car warms up, and
then the thermostat opens and it goes down to the middle of its range where
it's supposed to be.  What would be nifty would be something that would draw my
attention to that gauge if the reading stayed up near the redline for more than
a few minutes.  

How about a heads-up display?  And ONLY display those instruments that are
indicating out of "nominal" range.  Mount a backlit LCD atop your dash, wire it
so the display is mirror-imaged, and read its reflection off the windshield. Of
course since both the inner and outer surfaces of the windshield will reflect
you will get some ghosting in the vertical dimension.  This is, I think, why
the fighters with heads-up displays have a special piece of optic surface just
for this purpose (with an antireflection coating on the far surface). 

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@dcs.simpact.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
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