Raj has posted a tutorial about a DIY photoplethysmographic sensor for measuring heart rate from fingertip. “The sensor measures the little variations in the blood volume inside finger tissue and outputs a digital pulse which is synchronous with the heart beat.” The sensor used in this project is TCRT1000, which is a reflective optical sensor with both the infrared light emitter and phototransistor. The sensors output goes through several stages of signal conditioning using a MCP6004 op amp before its fed into an oscilloscope or similar device.
You can find the schemtic and other project info in Raj’s post on EmbeddedLab.
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