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Ear clip pulse monitor - interfacing?

Discussion in 'Electronic Design' started by Michael A. Covington, Nov 13, 2004.

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  1. Does anybody know the specifications of the standard ear-clip pulse monitor
    that is used with exercise machines?

    I know in general terms that it contains an LED and a light sensor (CdS?
    photodiode?) for a wavelength that is strongly blocked by oxygenated

    And I know about Jim Thompson's earpulse.pdf circuit, which does a very nice
    job of working out the filtering. But did he actually use the type of ear
    clip that is used with exercise machines? If so, on the plug, which pin is
  2. Jim Thompson

    Jim Thompson Guest

    I did that design in the early '70's, before LEDs were common, so
    that's an incandescent lamp (as marked on the schematic) in my design,
    but an LED should work fine.

    The output is TTL/CMOS-compatible (as also noted on schematic :)

    Pin-out? You got a DVM with an ohms setting ?:)

    ...Jim Thompson
  3. Yes, but I don't know if the sensor is a phototransistor or a CdS cell... I
    could work it out, of course. I was just wondering. Exercise machines all
    seem to use the same ear clip, and I thought the circuit would be out
  4. "Michael A. Covington"
    This was part of an undergrad lab exercise, long time ago ... a course
    in digital signal processing using TTL.

    I was involved in early market planning for a pulse reading exercise
    electronic system for Diversified Products (also a long time ago ... 197x).

    Then there was a system for a bicycle roller ... only worked on the
    software for that one, used a cardiac strap.

    In the 90's I worked on the optical detection of glucose by looking through
    the skin. Easy to do on potatoes, hard to do on people.
    Any old wavelength works. In the v. old days it was a grain-of-wheat
    lamp and a phototransistor. I don't remember a CdS cell being used, they
    don't like heat and moisture and hermetic packaging is $$, and they are
    too big.

    The big problem is loosing the signal as the exercise moves it's head.
    But that is all clip design.
    I doubt if there is a standard, exercise equipment manufacturer's seemed
    to be a very paranoid lot.

    I would assume a modern devices uses an LED and a photodiode. With a
    diode tester you should be able to find a .7v and a 1.1 volt diode by
    testing the available leads.

    Cable limpness plays such a part in getting these measurements to
    work reliably that I would not be surprised if only three leads are
    used: common, LED, PD.
  5. Jim Thompson

    Jim Thompson Guest

    Somewhere around here in my junk box I have one of those earclips, but
    I doubt that today's versions are the same... like I said, it was
    around 30 years ago.

    ...Jim Thompson
  6. Rich Grise

    Rich Grise Guest

    A photodiode will ohm out drastically different from a CdS. And as others
    have mentioned, it'd be quite unlikely that they'd use one of them in
    that sort of app anyway.

    So you're ohming out two diodes. The one with the greater forward
    drop is probably the LED. Then, pick suitable resistors, borrow a
    couple of meters, and find out what happens!

    Good Luck!
  7. Trevor

    Trevor Guest

    If it helps "Modern" oxygenation meters (which also measure pulse)
    pass two different light freqencies alternately thru the skin - they
    are then able to self calibrate and measure the quantity of oxygenated

    I can't remember the frequencies involved, but if u want I could dig
    out the info when I'm back at work.
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