Collin’s Lab: Infrared heart sensor

The beating of the heart is often recorded via sound or simply with touch – but there’s also another interesting and somewhat lesser known option – light. An infrared emitter/detector pair can be used along with a programmable microcontroller, to effectively visualize the effects of the human body’s hardest-working muscle.
http://blog.makezine.com/archive/2009/11/collins_lab_infrared_heart_sensor.html