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EEVblog #1334 – Mystery Dumpster Teardown

Mystery dumpster teardown time! With the most amazing mechanical mains power switch you’ll ever see! ...


  1. I remeber the in first video you mentioned that left and right were swapped. Did you try that? I assume the system is designed to use the fact that your left arm is closer to you heart than your right arm.

  2. I’m guessing they are doing some voltage to frequency conversion to pass the analog signal over the optocoupler. You might be able to reverse the process with the FFT function on your scope. Another option for demodulation might be to use a 4046 PLL and probe the VCO control voltage.

  3. I guess a lot of the mush between pulses is 50Hz mains frequency, no?

    I’ve got an 8 channel Contact Precision EEG / ECG amplifier and when this is connected with chest electrodes you get similar mush on a scope until you switch in the 50/60Hz analogue filter it contains.

    In your case the filtering is probably done in the DSP.
    Texas Instruments have some interesting Application Notes for their Instrumentation Op-amps and processors relating to ECG and healthcare (even if your board is not using them):


    In particular take a look at this one:



  4. Perhaps I’m mistaken, but I always understood that the acronym for electrocardiogram was actually taken from the German – EKG.

  5. I wonder if using a analog scope would be better due to not having any digital sampling messing up the changing square wave?

  6. Looks pretty clear to me that the optoisolator output signal is the ECG waveform encoded with either PWM or FM.

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