> Transmission has a VSS (speed sensor) which the ECU needs.  It
> apparently puts out a sine wave of increasing frequency AND amplitude
> based on vehicle speed.
>
> ...
>
> The speedo spec (when calibrated correctly against a GPS), says
> it's using approx 8000 pulses per mile.  This translates to 400 Hz
> at 160mph.

This could be separate sensor, or it could be right off the alternator.
Since the signal amplitude increases with frequency, a little low pass
filtering works to your advantage.  400Hz is also low enough that you can
put a reasonably large resistor in series with the comparator input.  In
your case, you can even put a small capacitor to ground at the comparator
input.  Try 1Mohm and 1mF, assuming the comparators has significantly higher
input impedance than 1Mohm.  1Mohm nd 1mF gives a rollover frequency of
about 160Hz, or about 64 miles/hour.  Below that speed the capacitor won't
do much, and above that speed it will start to hold the AC voltage roughly
constant, which gives you better noise immunity working with the hysterisis.
You do have some hysterisis, right?


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body