Heart rate is one of many important physiological parameters. It can be an indicator of workout intensity, stress, and even decease. With heart rate information you can do many interesting signal processing experiments like detecting its variations, or even fuse with other physiological data. There are several ways of getting heart rate data. One well known method is by using LED and its light pass variations through finger (pulse-oxymetry). Ohhitch have tried to attache piezo sensor to finger and found out that it detects heart rate very well.
Piezo sensor was connected to Arduino analog input which reads voltage differences due to blood flow pressure changes. This is enough to get reliable data. Simple Arduino code reads analog value and applies simple thresholding procedure which calculates heart rate in bpm.