For heart rate detection I have used the clamp on ear lobe type detector. They are very inexpensive and can be purchased at comercial outlets. These are the type of sensor used on the battery powered devices you would find at a bicycle shop for a bicycle rider heart monitor. There is similar equipment sold at exercise equipment shops. These clip on units have no electrical connection to the body and therefor are quite safe. They work by shining a red LED through the skin of the ear lobe and detecting the amount of light passsage via a photo transistor on the other side.

The signal from the detector requires some serious signal conditioning. This includes gain, automatic level control of the baseline or average level of the signal and a notch filter to get rid of the power line frequency of 50 or 60 Hz. (Lighting all around us effects out bodies more than most of us will ever know).

Once the sensor waveform is conditioned it can be driven into a comparator or high gain buffer clamps the waveform at logic levels. This can then be placed into a normal input pin. A relatively slow speed timer interrupt can be then used to sample the input periodically to see how many interrupt times the signal is low and then high and then back low. It can be "low speed" becasue even a fast heart rate at 120 beats per minute is at just 2Hz !! I have used a 100 Hz sample with good success.

Good Luck
Michael Karas