I can't find the original thread, but someone recently asked about data acquisition for an excecise bicycle. The OP wanted to measure heart rate and RPM. If he's still reading this NG, he may want to take a look at the December 2004 issue of Circuit Cellar magazine, which has an article by Jeff Bachiochi called "Light-to-Frequency Conversion (Part 1)". In it, he describes a nifty approach to measuring not only heart rate, but also blood oxygenation using a device with both red and IR LEDs, and comparing the ratio of the transmission to get oxgenation.
Hope this helps!
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
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