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 Heather’s Heartbeat Raw Signal

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