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PulseFit-Pulse Sensor with AutoAdjusted Threshold

This is a simple heartbeat indicator that automatically calibrates to a new user and pulses LEDs when it detects a heartbeat

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All the DIY pulse sensors I have used previously have some sort of threshold that has to be adjusted to each new user. The threshold is tuned for each person or is generalized so that it fits any user. I haven't played with one (at least in the DIY space) that has an auto-adjusting threshold built in the hardware. I don't know how well it will work, or even if it will, I was just curious.

The circuit incorporates the basic pieces of a typical pulse sensor. It contains an LED to shine light through someone's finger, a photodiode to detect the light that passes through the finger, a transimpedance amplifier to amplifier the current from the photodiode, and an active bandpass filter to amplify the signal and filter out the noise. My main addition is a peak detector circuit at the output of the band-pass filter and a comparator.

*****NOTE: This is NOT a medical device

*****NOTE: This is NOT a medical device*****

Demo!

Using a diode and capacitor pair, the peak detector captures the peak AC amplitude of the pulse waveform and holds the voltage (because of the capacitor). When we place a resistor in parallel with the capacitor, the resistor allows the capacitor to discharge at a rate equal to the time constant of the RC circuit. If we tune the time constant correctly, the peak detector circuit will hold the peak long enough to distinguish the peak pulse amplitude from secondary peaks due to the dicrotic notch. Also, because of the discharge of the capacitor through the resistor, the peak detector does not hold the voltage indefinitely, but it adjusts to the changing peak AC amplitudes of the pulse waveform due to stronger or weaker pulses.

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    For step-by-step instructions, please visit the project's Instructables page.

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