speedometer help noobie

Discussion in 'The Projects Forum' started by electric77, Mar 21, 2016.

  1. electric77

    Thread Starter New Member

    Mar 18, 2016
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    I have a digital speedometer that is part of my ATV

    i want the data from the speedometer about the sped of my ATV to be available to a Raspberry Pi or arduino.
    not sure how to do so.

    what i have planned to do was take apart the speedometer wires, locat the speed signal wire then convert it to a number in m/s or any units i want.

    i need help with what to use if anyone has any suggestion.
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    It shouldn't be necessary to disassemble the speedometer to get at the speed pulse, just pick it up from the sensor before it goes into the speedometer. Once you have the incoming pulses it's possible to convert these to a useful number by either measuring the time between pulses or by counting how many pulse are received within a fixed time frame. The details of how you convert this number to a speed value depend on knowing how the incoming speed pulses relate to road speed.
     
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  3. Marcus2012

    Member

    Feb 22, 2015
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    I'm not sure but you might need to find out either wheel diameter/circumference or wheel turns per mile in addition to rotations per minute (pulse signal).
     
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  4. electric77

    Thread Starter New Member

    Mar 18, 2016
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    1
    sweet,
    i was watching a video earlier about people adjusting their speedometer reading like that
    so yes, i got the signal wire and i will get the circumference to get distance of one revolution.

    but how can i automate this, im trying to getting the speed as an input to raspberry pi and use that for something other stuff i have as a part of my project.

    looking for exact component (what can I use to get the speed), or is it just a code?

    and best method to find pulse wire would be a volt meter? (black attach to a neutral part and red wire test each pin while car is accelerating)

    my speedometer has 3 wires.
     
  5. Marcus2012

    Member

    Feb 22, 2015
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    If it has 3 wires it is most likely you have 12V(or whatever constant voltage supply you have), Ground, and pulse. Similar to a 3 pin computer case fan. Ground you can presumably testing for by checking for continuity between the pin and the chassis (if it uses the chassis for a ground return pathway). Once you have this you have work out the constant supply by measure the voltage differential between your ground pin and the other two until you get a value that corresponds to it's supply (with the ignition ON, if applicable). The third pin, if connected to a hall effect sensor will lack both voltage when stationary and continuity between ground.

    As for how you can go about measuring the signal in a moving vehicle(or on a rolling road) and how to interpret that data, I'm not sure sorry.
     
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  6. electric77

    Thread Starter New Member

    Mar 18, 2016
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    its an ATV, its not an issue testing the speedometer while running, my only issue is, how do i connect the signal wire to my raspberry pi and get useful data about speed
     
  7. Marcus2012

    Member

    Feb 22, 2015
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    I literally haven't done anything close programming in years but I'd imagine you'd need to compile some sort of analog to digital converter to interpret when and how many times that signal is going high over a sample period defined by your internal clock and thereby determine revolutions per minute (bear in mind your transmission may have multiple magnets i.e multiple pulses). Then you'd have to apply your constants of circumference or wheel turns per mile to calculate the distance covered in that time which is essentially your speed when multiplied over a larger timeframe. Displaying that output visually, I guess would be easiest on a 7 segment display. But as I say I've never done this and it all really depends on how comfortable you are programming all of this. And there are probably lots of steps I've missed.
     
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  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Road speed is calculated in the code. It should be possible to identify the speedometer pulse using a meter in the way you describe or from the wiring diagram, if you have one.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Depending on the speed sensor, its output pulse might need to be attenuated so that it doesn't fry (bake? :) ) the Pi.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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