Simple circuit to identify an 1Hz signal

Discussion in 'General Electronics Chat' started by bug13, Nov 23, 2014.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    I got an digital IO which has 3 state:

    • Always high
    • Always low
    • 1Hz square wave
    Is there a simply circuit to ID the 1Hz signal? Like when the IO pin is outputting the 1Hz signal, the circuit can change state, but when the IO pin is always low or always high, no state change.

    Thanks guys!

    PS: I am trying not to use a MCU.
     
  2. JWHassler

    Member

    Sep 25, 2013
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    look for a "retriggerable one-shot" or "missing-pulse detector"
     
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  3. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Your solution could be something as simple as a led on the output if all you need is a visual indicator. More info would help.
     
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  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi Gerty, What other info do you need?
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    +1
    Just stick led on it.
    LED off, pin is low.
    LED is on, pin is high.
    LED is flashing, pin is putting out 1 Hz square wave.
     
    Last edited: Nov 23, 2014
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Ah I see, I am sorry to confuse you guys.

    The circuit I want is the output stay high when the input is 1hz square wave.

    The output is low when the input is always high or alway low.

    This should clear up the confusion. Sorry about that.
     
  7. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Hi Gerty, What other info do you need?
    Voltage level and what you need from the "indication" for starters.
    As shteii01 posted above is what I had in mind, would that suffice for your needs? Or do you need a signal, that can be logged, versus visual indicator?

    3 posts at once!!
     
  8. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi

    My post on #6 should clear up the confusion.

    But here are the extra info:
    Low = 0V
    High = 3.3V
    Sink or source = ~5mA (but I wantto draw as little current as possible)
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That sounds like a typical watchdog timer or charge pump circuit.

    I second the missing pulse detector, a 555 for e.g.
    Max.
     
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  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    To review.

    Input is low, output is low.
    Input is high, output is low.
    Input is 1 Hz square wave, output is high.

    I am thinking some kind of frequency to voltage converter. I have not worked with them so I think it is best if someone with more experience chimes in.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    With a 555 missing pulse detector, (watch-dog timer) if the input is via a input capacitor, only the pulses are received by the 555, any permanent high or low the timer times out.
    Pulses keep it in reset.
    Max.
     
  12. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    And by the timer times out you mean... that the output of 555 is low (0 volts)?
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes.
    Max.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    I vaguely remember a trick to do that with an XOR gate and a C/R integrator.

    If the signal remains static, the capacitor on one input charges via the resistor and becomes the same as the direct input - so the output goes low.

    With the signal toggling, the integrator cant keep up - so you get periods of different input states, which results on logic high out.

    If you need a steady indication - just use a second integrator.
     
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  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I guess you cannot expect a very fast response time between status changes.

    How about a simple capacitor coupled inverting op amp amplifier with rail-to - rail input/output and a low reference voltage.

    Here is a filtered, capacitor-coupled amplifier. It will turn on for about 0.5 seconds when signal goes high, but no divice will be able to tell the difference between a high and a 1 Hz signal for the fist half second.

    image.jpg

    And one short pulse when "high"

    image.jpg
     
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  16. ScottWang

    Moderator

    Aug 23, 2012
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    1. Using a op amp integrator -- 1 second RC delay time to filtering the quick changing status from low to high and from high to low.
    2. Using a op amp differentiator circuit -- to pass the 1Hz signal.
    3. Adding a 1N4148 diode and a capacitor to do the rectifier and filter for 1Hz, then you will get a high.
     
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  17. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    It's hard for me to understand without a schematic, could you draw a simply one up?
     
  18. ScottWang

    Moderator

    Aug 23, 2012
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    You should modify the title.
    It just an idea, I don't think it's hard for you, I already give you the linking page and description for purpose, it may draw it.
     
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  19. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    • I am not sure how to change the title.
    • What is the reason you want to integrate the square wave input?
     
  20. ScottWang

    Moderator

    Aug 23, 2012
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    You could ask moderators to modify the title.

    The integrator is not for the square wave, your mentioned has three states, what I concerned and my original idea is to filtering the changing states from low to high or from high to low:
    1. The io changing from low to high and stay at high.
    2. The io changing from high to low and stay at low.
    3. 1hz square wave, it will including the status as above 1 and 2.

    I was in the countryside and only the cellphone in my hand now, so, i can't do any test, the problem is in the item 3, separate them the problem will be solved.
     
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