On 6/12/08, Robin D. Bussell <RobinB@excelerate.info> wrote:
>
> Hi Folks,
>      It looks like I may just have found the perfect excuse to get a
> PIC into my car :)
> The speedometer died the other day, investigation revealed the cause was
> the failure of a drive gear deep in the gearbox that will take lots of
> cash to get replaced, a fair fraction of the car's worth in fact :(
>
> It's an electronic speedo drive so I need to synthesise a signal to feed
> it. One source of speed related information would be the anti lock brake
> system wheel sensors which are of the magnet, coil and toothed wheel
> variety, outputting 46 pulses per revolution at 0.5 - 0.7V (so I'm
> told).
>
> The speedo wants 2,458 pulses per kilometre. This works out at dividing
> the wheel sensor frequency by about 2.367 with a maximum frequency from
> the wheels of 320Hz at 150MPH (no I don't plan to go that fast! Might as
> well design to the max speed though).
>
> So I was thinking that for this purpose I could do this with a small
> pic:
>
> Take 1000 "1" bits and distribute them evenly over a 2367 bit string
> with everything else being "0", (so we're looking at a 296 byte table)
> every time a pulse comes in from the ABS, read the next bit in this
> string and set an output pin appropriately. End result should be a
> signal that is (on average) the input divided by 2.367 .
>
> Can anyone see any problem with this algorithm in this application?
>
> I daresay I could happily cut down the table length too, I only need a
> maximum error of 5% or so I guess.
>
> All thoughts welcome!
>
> Cheers,
>     Robin.
>
>
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 What year vehicle, I did a speedometer by using an OBD II reader from
www.obdpros.com  speed is available by sending 010D to the interface.

 The folks at OBDPros helped me out with interfacing the unit with a PIC.

This eliminates having to hardwire into sensors but in retrospect I guess I
could probably have gotten the problem fixed for a little bit more than what
I spent on this solution


Mike
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