January 2005, Issue 174

Light-to-Frequency Conversion (Part 2)
Pulse and Oxygen Content

Last month I left you with a graph of sampled data from a project that uses a TAOS TSL230R programmable light-to-frequency converter to monitor the light transmitted through living tissue. You learned that the heart’s pumping action moves blood through the body via its distribution system of arteries, capillaries, and veins. The oxygen collected in your lungs passes through your entire body in this way. When your heart muscle contracts, it increases arterial pressure as blood cells are forced out into the arteries. The arterial system, which is strong and flexible, slightly expands and contracts during pressure changes. Although you can feel this pulsation best at specific points on the body, it takes place everywhere.

Your bones and tissue block (or absorb) much of the light passing through your body. For the most part, a constant (although small) portion of light will pass completely through. This light intensity will be modulated slightly by the arterial system as it pulsates and dynamically changes the amount of light that is able to pass through it. This project monitors light passing through the body in an effort to extract heartbeat information from the small dynamically changing portion of light.

As I explained last month, the TSL230R eliminates the need for designing high-gain analog circuitry to amplify the output of a photodiode. This analog circuitry is extremely susceptible to external noise and it requires an A/D converter to get the signal into a microcontroller for data analysis. The TSL230R’s frequency output eliminates this messy front end and is easy to attach to any microcontroller thanks to the TTL compatible I/O.