Digital Speedometer

[Vitek, Joseph]

> Date: Mon, 10 Apr 2000 10:18:46 -0600
> From: cwagner at info2000.net
> Subject: Digital Speedometer
>
> Does anyone have anything that could help me in building a digital 
> speedometer for use as a HUD?  I am not worried about the 
> reflective surface right now, I am more interested in the electronics 
> part of it.  

I've had the whole custom digital dash on the back burner for some time now.
About 15 years ago (I'm 35 now), I designed a discrete logic speedo that
worked perfectly in my big block Impala (oh, the performance years -- youth,
i mean) and was easily calibrated for different tires/ wheels or gearing,
etc. I had forgotten about it until last year when I started to get the itch
to do a custom dash for my '87 Cutlass. I found my old design on paper and
put it together again from my original notes. It worked! Well, it should
'cause it worked 15 years ago. Don't know what happened to the original
unit, probably got raped for parts. Anyway, I have two Cultii now, an '81
with a 455 and the '87 with a 403 (both injected, of course). The '81 is
getting the total race interior (sheetmetal, cage, big analog gauges, etc.)
and the '87 is getting the as yet not quite designed digital dash. I've had
a number of ideas that I've kicked around like:

the old RCA 3161 and 3162 A/D and display drivers for digital gauges (have
complete design on this)
PIC microcontroller with onboard A/D and display drivers (started design)
PALs, GALs, FPGAs, etc. and display drivers
68HC11 and flat panel backlit LCD display (expensive)

Lest, I digress. The original question was about a digital speedo. If it is
on a car, 4 small magnets can be epoxied onto the driveshaft 90 degrees
apart. Then use a Hall effect sensor mounted on a bracket in line with the
magnets. This is your speed pickup. It's easy to calculate the pulses/second
for a given speed knowing your tire diameter and gear ratio. Then you just
need to count pulses for a set time period and then do a little math to give
the speed directly. If you use some high brightness seven segment LED
displays, you can get a pretty good reflection on the inside of the
windshield. You have to mirror the display once before it is reflected off
of the windshield so it reads unmirrored.

I would also like some ideas on the digital dash idea as I have the design
experience but am lacking any more novel ideas.

Please feel free to contact me directly. I really am into working on this.

BTW, I'm on the digest.

Joe Vitek
RF Design Engineer
Terion, Inc.
321.757.5962
jvitek at terion.com 
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