how is heart beat counter working?

Thread Starter

theasus

Joined Sep 12, 2009
16
I couldn't understand this circuit's operating.
Could you help me about this circuit details?

 
Last edited:

hgmjr

Joined Jan 28, 2005
9,027
I couldn't understand this circuit's operating.
Could you help me about this circuit details?
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
 

t06afre

Joined May 11, 2009
5,934
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
 

jpanhalt

Joined Jan 18, 2008
11,087
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:
The absorbance of oxyhemoglobin and deoxyhemoglobin is the same (isosbestic point) for the wavelengths of 590 and 805 nm
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
 

Thread Starter

theasus

Joined Sep 12, 2009
16
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...
 

jpanhalt

Joined Jan 18, 2008
11,087
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
 

Thread Starter

theasus

Joined Sep 12, 2009
16
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
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???
 

jpanhalt

Joined Jan 18, 2008
11,087
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
 

ELECTRONERD

Joined May 26, 2009
1,147
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
 

bluebrakes

Joined Oct 17, 2009
252
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.
 

Thread Starter

theasus

Joined Sep 12, 2009
16
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
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?
 

jpanhalt

Joined Jan 18, 2008
11,087
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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