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Measuring Heart Rate with a Photoplethsymograph

Measuring Heart Rate with a Photoplethsymograph. David Luong and Heather Jones. Context and Importance. How do we measure heart rate? Counting beats is slow and inaccurate Photoplethsymograph is quick and non-invasive. Theory. Shine a light through finger onto CdS cell

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Measuring Heart Rate with a Photoplethsymograph

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  1. Measuring Heart Rate with a Photoplethsymograph David Luong and Heather Jones

  2. Context and Importance How do we measure heart rate? • Counting beats is slow and inaccurate • Photoplethsymograph is quick and non-invasive

  3. Theory • Shine a light through finger onto CdS cell • Resistance of CdS cell depends on amount of light hitting it • Amount of light hitting CdS cell depends on the amount of blood in the finger • Amount of blood in finger varies as heart pumps blood through body

  4. Design Process Diagram courtesy of E. Cheever

  5. Data Processing • Band pass filter • 120 Hz noise from light • DC offset of sensor • Signal at about 1 Hz • Amplification • Original signal < 100mV • Need 0-5V signal for A/D • Use Vcc/2 as virtual ground to move signal to center of range

  6. Data Processing: Before and After David’s Heartbeat Raw Signal Heather’s Heartbeat

  7. To Digital and Beyond • ADC0804 used to convert signal • Output of ADC goes to PIC • Program to calculate heart rate • Calculate dynamic range using min and max of signal over 3 seconds • Start counter at high threshold • Stop counter after passing low threshold and returning to high threshold

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