Design
Being a wearable computing device, the hardware all is contained within a ballcap. Cables are run along the band of the hat and velcro straps are sewn into the back to hold larger chips. The optical pulse sensor is attached to an adjustable foot on a wooden arm to ensure sufficient pressure is applied to the temple. The LEDs are run over the top of the bill and bend down into the field of view of the wearer.
The core of the project is detecting, measuring, and calculating the user's heart rate. The sensor initial was intended to be constructed using a simple red LED and photoresistor. By placing a thumb over both the LED and the photoresistor we can, in theory, detect a pulse. As the heart pumps, blood pressure increases, filling the vascular tissue in the finger with more blood. This increase in blood makes the tissue more opaque, reflecting back more light. The photoresistor detects this change in light.
We built a small optical heart rate sensor out of these components and tested the output using an oscilloscope. The results from these tests, displayed to the right, showed that the LED was not bright enough and the photoresistor was not sensitive enough. The difference between background (no light, no thumb) and the LED with photoresistor (LED and thumb) tests are minimal. No repeating patterns can be seen. The bottom waveform shows the results using a 100 lumen flashlight in place of the LED. A repeating pattern can be seen indicating a heartbeat. With these results we decided it was not reasonable to construct our own optical pulse sensor.
For these reasons, we used a commercially available optical pulse sensor with a much brighter LED and much more sensitive photoresistor. We were able to get much more definitive results without having to filter out excessive amounts of noise. To the right is a mapping of the sensor's PWM output over time. It is clear that the new sensor gives much more precision and accuracy than our initial version.
