What type of vehicle is it?  Have you really seen (on an oscilloscope, et=
c)=20
that it is a square wave?  I say this because most new cars use this spee=
d=20
sensor for the cruise control and FI strategy, and usually use a VSS whic=
h=20
puts out a sine-wave whose amplitude increases with speed.  Aftermarket=20
speedometers generally have a speed sensor that puts out a 12V square wav=
e. =20
Still, it is possible that you do have a square wave, but try to verify t=
his=20
first.

Assuming a square wave,  you should be able to simply send this signal in=
to a=20
transistor switch via a resistor (some tens of kohms) and have no ill=20
effects, but if you really want hi-impedance, you can use an op-amp.  If =
you=20
have a sine-wave, then you can use a comparator to square up the signal.

Either way, you can verify that you don't have any ill effects by pulling=
 the=20
codes after driving some miles, as most ECUs will flag a VSS-signal probl=
em.

Cheers,
-Neil.



On Tuesday 23 September 2003 07:30, Hulatt, Jon scribbled:
> Hi all,
>
> For my next project, I want to initiate some action when my car exceeds
> approximately 10mph. The "speedo" sensor is some kind of proximity swit=
ch
> on the output end of the gearbox/diff. So, the output it a 12v ish squa=
re
> wave, of frequency proportional to speed. My multimeter measures the
> frequency at about 20 Hz at 10mph.
>
> So, to measure this frequency, my idea is to use the CCP, in capture mo=
de,
> with an appropriately prescaled timer. My Capture ISR will increment a
> counter each time the CCP event fires, if the timer represents a (perio=
d)
> shorter than 40ms (ie, faster), and, when the counter gets to an
> appropriate number, perhaps 20 ish, I'll inititiate the other code. Ide=
a of
> using the counter is to filter out any bounces or other muck in the lin=
e.
>
> Leading to my first question- does this plan seem like a workable idea?
> I've not much experience with a) pic and b) the automotive envorinment.
>
> Secondly, is the question of how to interface the 12v square signal wit=
h
> the pic? I'd thought of using an optoisolator, but is there a better, o=
r
> simpler way of protecting the pic from a) getting 12v not 5v and b) any
> dirty spike type things? I think I need a relatvely high impedance, bec=
ause
> the signal goes into the ECU, so if I steal too much of it, I may make =
the
> car rather ill.
>
> Appreciate your thoughts.
>
> Jon

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