I am going to use the HRM-2511E fingertip pulse sensor manufactured by Kyoto Electronic Co., China in the prototype of this project. I have worked with this sensor before and I have found it very easy to use, and provides stable output. The HRM-2511E sensor body is built with flexible Silicone rubber material that fits nicely on the top of a finger. Inside the sensor case, an IR LED and a photodetector are placed on two opposite sides and are facing each other. The necessary connections to the LED and photodiode pins are available through a 3.5mm stereo audio connector.

HRM-2511E pulse sensor