Re: Speedometer - transparent first-surface reflector needed!

["mike mager" <mikemager@hotmail.com>]

cwagner asked the GM ECM list: (gmecm@diy-efi.org)

> Does any one have any good ideas on making a digital
> speedometer, preferably one that can be mounted in a heads-up
> situation?  More gadgets to fight with, but they are fun.

Alright!  I've got to 'grab' this one!

I am a guy that is completely fascinated by all types of instrumentation.  
In 1972, or so, back in high school, I was completely awed by a Detroit 
concept vehicle with full digital read-out instrumentation (probably filled 
the trunk with electronics?).  That was the beginning . . .

I went on, by 1978, to learn how to build such instruments myself, using 
'discrete' MSI ICs (very bulky, un-calibratable, unreliable, 
power-consuming, heat-generating and temperature-sensitive, and just 
generally bad).  I almost figured out MCUs and CPUs in 1981, but it was so 
exotic and expensive, that it took me a few years to get back to it, and 
then it was, "Oh, yeah, I see how it works, that's easy!".  Now I think 
nothing of buying PIC in the UV EPROM package, doing a small bit of assembly 
language coding, a small bit of hardware, and having a working circuit.  
Digikey works wonders.

I have built a fairly complex instrument system for automotive use, and it 
certainly is no big deal.  I am successfully procrastinating the (absolutely 
necessary with this level of complexity) systems analysis for a very 
complete, complex, complicated, and worse system (but I think of it often).

Somebody else may have already been done answering by now, but . . .

I like to say "head-up display", because I only have one head, but most 
others refuse!  I did build a head-up display for my speedometer a few years 
ago, and I found a (known) problem:
I had my development board loose on the top of the instrument panel, and one 
night I noticed a reflection of the three ~1" green seven-segment LEDs in 
the windshield - wow, HUD!;  I went in, connected a one-bit input switch, 
and changed the code to read it, and to select my original seven-segment 
bitmap, or to select a mirror-image bit-map - you may have seen this done on 
the police RADAR displays, to be readable in the rear-view mirror.  There in 
a short time, I found the problem.

The problem was that the windshield has (at least) four reflective surfaces 
(think of a first-surface mirror for LASER vs. a regular mirror, and add 
laminations), and I got way too many reflections - but it did 'work'.  A 
'real' HUD uses a transparent first-surface 'reflector' adhered to the 
windshield - now, where can we get one?  I see that like the 1992 or so 
Nissan 240 SX had HUD, with that proper reflector, and that they are now (I 
believe) out of production;  I was told that they were too expensive because 
of the 'special' windshield, so, that is not a source of a separate 
reflector.

Please give more specific details, if you have any (I could hardly stand 
another project, but . . . ), and maybe we can each build one.

We need to find a transparent first-surface reflector!

Mike
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