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.

Thanks

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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.)

    www.chem.ucla.edu/.../an1003

  • I appreciate the thorough response. The circuit works well more or less. I've used a potentiometer to help move the range of the offset to be centered around 2.5V. I'll double check my assumptions using a voltmeter.

    Thanks!

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