Detecting frequency threshold

Discussion in 'General Electronics Chat' started by markdem, Jun 30, 2016.

  1. markdem

    Thread Starter Member

    Jul 31, 2013
    74
    38
    Hi All,

    I have a motor with a tacho output. The frequency changes depending on the rpm between 0hz to 360hz.
    What would be the best why to detect when the frequency is lower then some setpoint, lets say 200hz?
    Accuracy only needs to be about +\- 20hz.

    I could easily do this in a PIC, but all I really want here is to have a 5v output when the frequency falls below the setpoint.

    What would be my options?

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
    16,296
    6,807
    This is how to get a voltage that changes with frequency. More frequency = more voltage. Follow this with a comparator to decide what voltage will cause switching and design for a high output when the frequency is too low.
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    Or you might look at LM2917 designed for the purpose.
     
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Or, a LMC555 as a missing pulse detector.

    Ken
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Last edited: Jun 30, 2016
    #12 likes this.
  6. markdem

    Thread Starter Member

    Jul 31, 2013
    74
    38
    Thanks guys.

    I think I will go with #12's solution. Nice and easy. Will keep the LM2917 in the bag, might come in handy..

    Thanks
     
  7. Techno Tronix

    Member

    Jan 10, 2015
    140
    10
    Also LM2907 /LM2917 can be used for frequency discrimination.
     
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