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If RPM, then power my device

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  • If RPM, then power my device

    I'm not an engineer, but am researching a project around OBD-II. Basically, just want to know how I could read RPM and based on a threshold, power a 12v device. If RPM dips back below that threshold then I'd want to drop power.

    And this application would have to be 99.9% reliable on newer vehicles. How doable is this project and can it be done relatively cheaply?

  • #2
    it can be done in RideRunner or Centrafuse right now using a combination of their OBD plugins and Fusion Brain plugins.. but you're not going to get anywhere near that 99.9 percent reliability. I doubt you'd get that using professional equipment Senior Tech Blogger (Want a product reviewed? Contact me.)
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    • #3
      Use an ELM or compatible interface and poll for RPM (mode 1 Pid c) by sending "010C\r" to the vehicle interface. Monitor the response and parse the two bytes after the "41 0C" bytes and there will be the RPM value. Works on all OBD-II compliant vehicles. I haven't seen a vehicle yet that doesn't support RPM this way, although a Prius will often return "0000" when running electric


      • #4
        Sinclairvital, you answered the question pretty well. I'd already read much on the forums about doing this and I have a fair degree of confidence that I could purchase an ELM-based product for no more than $30 to accomplish this manually.

        That said, I'm looking for something that's totally integrated into a lightweight cable, relatively inexpensive in quantities, and completely automated. That is, the blunt end of the cable would simply be 12v if RPM threshold is hit and 0v if it's not. No computers or analyzers in the mix.

        How would you accomplish this? And am I dreaming of something that's not reliable or cost effective? (need total cost in hundreds of units to be less than $30)


        • #5
          We do custom hardware design, but $30 per unit for a few hundred units is not realistic. You would need to order 10,000 units to get close to that price point.

          If you plan to leave the cable permanently plugged in, there is also the issue of draining the battery unless your circuit has a sleep feature.
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          • #6
            Here are the prices for the elm chip alone (just the IC):

            with bulk pricing that drops a little bit but still not enough to reliably create a $30 product. In addition to this you will need a separate IC to communicate with the elm327IC and control the 12v source. At best your talking about $50/each in bulk.

            You could also try using the Chinese knock off elm chips, quality hasn't been that big of an issue, just speed (which you don't really need if polling a single PID). I don't have any sources but apparently the knockoff IC is around $10.
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            • #7
              Originally posted by WhipClash View Post
              How would you accomplish this? And am I dreaming of something that's not reliable or cost effective? (need total cost in hundreds of units to be less than $30)
              What vehicles are you looking at targeting are you planning on all OBD2 compliant vehicles or are you primarily targeting newer vehicles (2009 onwards?)

              How much current would the device that you are planning on plugging into the blunt end draw?

              OBdPros Scantool


              • #8
                Whipclash, assuming you have a device that reads the rpm and does what it is supposed to do. What happens if the car has some form of problem and I have to remove your device because I need to connect a scanner to repair the car? Which may include test driving it.