how is heart beat counter working?

Discussion in 'Homework Help' started by theasus, Nov 5, 2009.

  1. theasus

    Thread Starter New Member

    Sep 12, 2009
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    I couldn't understand this circuit's operating.
    Could you help me about this circuit details?

    [​IMG]
     
    Last edited: Nov 6, 2009
  2. t06afre

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    May 11, 2009
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    Only if you show it to us ;)
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    As already indicated, we are hard pressed to imagine precisely what circuit you have in mind without a schematic.

    Hopefully you can attach one in this thread so that we can answer your question.

    hgmjr
     
  4. theasus

    Thread Starter New Member

    Sep 12, 2009
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    The schematic has been added.Could you help me please?
     
    Last edited: Nov 6, 2009
  5. t06afre

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    May 11, 2009
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    Your device seams not to be a heart beat monitor directly but a device for measuring Saturation of peripheral oxygen. As this level changes within a heart beat, you will be able to extract the heart beat frequency very well. It is done on regular basis daily in every hospital.
    You can use this jump point for more information. http://en.wikipedia.org/wiki/Oxygen_saturation
     
  6. jpanhalt

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    Jan 18, 2008
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    On the other hand, since it uses only a red LED, I suspect it is detecting the pulse of blood, not its saturation.
    From Wikipedia:
    By carefully adjusting the pressure of the device on the index finger (or any finger with a good pulse), it should sense a change with each beat.

    The rest of the circuit is a dual op-amp. One-half of which (inputs 5,6; output 7) appears to be operated as a comparator to give a pulsing output.

    John
     
  7. theasus

    Thread Starter New Member

    Sep 12, 2009
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    In this circuit filter is a high band pass filter.(Frequency=7.2 Herz) But hearth beat is about 60-100 per minute. And it is equal to 0.5 to 1 Herz.How can this signal passes this filter?
    I tried this circuit and it is worked but I couldn't understand in theorical.And I need to know its theriocal solutions.
    And I need your help.
    Thanks...
     
  8. jpanhalt

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    Jan 18, 2008
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    Are you referring to the function of C5? Remember, you are looking at the change in intensity and want to block DC. The rate of change for the pulse edge will be higher than the average pulse frequency.

    John
     
  9. theasus

    Thread Starter New Member

    Sep 12, 2009
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    I'm sorry I couldn't understand this sentence;

    "The rate of change for the pulse edge will be higher than the average pulse frequency."

    Could you explain this sentence in more detail???
     
  10. jpanhalt

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    You calculated a high-pass cutoff of 7.2 Hz (I have not double-checked that calculation). Since heart rate is about 1 to 2 Hz max (not 0.5 Hz, which equals a pulse rate of 30), I interpreted you were asking how that low a frequency could get through the high pass filter.

    If you look at an actual pulse pattern, such as here: http://www.cse.iitb.ac.in/~ajjoshi/modernPulse.pdf
    you will see that a typical pulse is not a sine wave. The width of the pulse is much narrower than one-half of the distance between pulses. Also, the pulse is not shaped like a sine wave. Thus, there are many frequencies in an arterial pulse that are higher than the cut-off you calculated.

    As an interesting physiologic experiment, does your device work on the thumb and other fingers?

    John
     
  11. ELECTRONERD

    Senior Member

    May 26, 2009
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    Hey theasus,

    It's a shame you removed the schematic from your first post, why? Would you mind re-posting it for us? I'm sure many here, including me, would like to observe it.

    Thanks,

    Austin
     
  12. jpanhalt

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    Jan 18, 2008
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    The schematic shows on my machine. The last edit date is 11.06.

    John
     
  13. bluebrakes

    Active Member

    Oct 17, 2009
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    interesting circuit. I never really thought about how those finger heat beat monitors worked before.

    probably because i expected them to be used in conjunction with the pads.
     
  14. theasus

    Thread Starter New Member

    Sep 12, 2009
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    As you said:"The width of the pulse is much narrower than one-half of the distance between pulses" Can we calculate the frequency of the pulse?I have read your document but I couldn't find the frequency of the pulse. How can i determine the frequency of the pulse which will be processed?
     
  15. jpanhalt

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    Jan 18, 2008
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    Hi theasus,

    I have been away for awhile.

    I think you are confusing the wave pattern for an arterial pulse with calculations that apply to a sine wave. The pattern of an arterial pulse (time domain) can be resolved into a number of sine waves (i.e., frequency domain) using Fourier transform (see: http://en.wikipedia.org/wiki/Fourier_transform).

    Once you get to that point, you can determine how much of each frequency will get through that high-pass filter. A nice discussion of the principle is in Horowitz and Hill (The Art of Electronics, 2nd ed., 1989) beginning page 15 (paragraph 1.07) and continuing through page 32, particularly notice exercise 1.13 and paragraph 1.18 (page 30).

    In brief, the arterial pulse wave form will have sine wave frequencies in it that are higher than the cut-off you calculated.

    John
     
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