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Measure RPM from PWM input (4 wire fan)

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  • Measure RPM from PWM input (4 wire fan)

    My apologies if this question has been addressed previously, I wasn't able to find any answers though.

    I'm looking to use a 4-wire fan, using a PWM output to control speed (which I assume will be straight forward), but also looking to "read" a PWM input for the purpose of measuring the fan's RPM.

    Is there a way to achieve this?

    thanks,
    Dave

  • #2
    The easiest way is probably with an Arduino and the Arduino plugin.

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    • #3
      Hi Automated,

      I guess I should have added, I have an arduino and the plugin.

      I only see that a PWM pin is an output. Inputs seem to be analog or digital (hi/low) only.

      Not sure how to process a pwm square wave input.

      Thanks,
      Dave


      Sent from my SGH-I337M using Tapatalk

      Comment


      • #4
        Measure RPM from PWM input (4 wire fan)

        Originally posted by dhalsall View Post
        Hi Automated,

        I guess I should have added, I have an arduino and the plugin.

        I only see that a PWM pin is an output. Inputs seem to be analog or digital (hi/low) only.

        Not sure how to process a pwm square wave input.

        Thanks,
        Dave


        Sent from my SGH-I337M using Tapatalk

        You would have to use the api sketch. You should be able to find a arduino example of measuring fan speed then use SendToHS to sent it to homeseer

        Comment


        • #5
          I wanted to update this thread with what I ended up doing.

          As I really just needed to see a confidence return of the fan's speed (and not an exact RPM reading), I chose to design a hardware solution. Using a LM2907N, I took the RPM signal from the fan (not PWM) and converted to a DC level. The LM2907N does most of the work, and converts frequency to voltage. I chose components to roughly give me a range for the frequencies involved to convert to around 0-5vdc. The nice thing is that even when the fan is "off" (aka 0% PWM input), it still spins a bit and thus will still provide a level above 0V. When the fan blades stop by means of finger, or loss of power, the frequency drops to 0, and as a result, 0Vdc after the conversion.

          So, I get rough feedback that if X% PMW is applied, X% is returned on the confidence line. IF 0% is experienced, the fan has stopped, jammed or died, an alert is sent.

          I've attached my napkin scribble for reference if anyone else is looking to achieve the same.
          Attached Files

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