Heartrate Circuit

I wanted to receive clarification on the behavior of the LED in the circuit found at the following link provided by ADI: (wiki.analog.com/.../alm-lab-heart-rate-mon). Should the LED be flashing regardless of my finger placement, and should it be picking up ambient light? Or should it only flash when placing my finger above the IR LED and Phototransistor? I see it flashing differently if I place my finger, but want to see how I can reduce the effects of flashing when not placing my finger.


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    •  Analog Employees 
    on Apr 18, 2021 12:08 PM

    Did you ever get this circuit working to your satisfaction? It's a pretty basic circuit, and while the QSD123 has a visible light filter, it's ability to reject ambient infrared is limited.

    Note this statement in the lab writeup:

    "The collector load resistor R2 of the photo transistor can also be adjusted to optimally center the signal."

    You could gain some insight into what's going on by connecting a voltmeter to Q1 collector, and adjust R1 and R2 such that the voltage is centered around approximately 2.5V when your finger is inserted in the optical path.

    In "real" applications, the LED current would be modulated (turned on and off rapidly), and the photodetector output measured in both the on and off state, and the difference taken. Thus ambient infrared light is rejected, and only the infrared transmitted from the LED is measured.

    Do a web search on "optical lock-in", here's the first hit that came up, see figure 3. (The chopping can also be done electronically.)


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    •  Analog Employees 
    on Apr 18, 2021 4:46 PM in reply to mthoren_adi

    Chopping or pulsing the emitted light to reject ambient background light is a well used technique in Pulse-Ox and heart rate monitors. However, implementing such a scheme in conjunction with the continuous time analog filters used in the circuit in this lab activity would be rather difficult without making major changes. The analog filtering is designed to mainly reject line frequency AC variations in any background light and only pass the approximately 1 Hz signal from the blood pressure variations as the heart beats. As was pointed out strong background light levels can swamp the reflected IR light from the finger and as suggested adjustment to the average value coming from the sensor to center it in the available power supply range will be required.

    You might well get better results if used in a more dimly lit room.

  • I will test this, and appreciate the clarification!

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