Abstract: In one aspect, a photoplethysmograph system to measure a user's heart rate includes one or more light-emitting diodes (LED) that provide a constantly-on light signal during a measurement period. The one or more light-emitting diodes are in optical contact with an epidermal surface of the user. The one or more light-emitting diodes emit a light signal into the tissue of the user, and wherein the tissue contains a pulsating blood flow. A light-intensity sensor circuit converts the reflected LED light from the tissue into a second signal that is proportional to a reflected light intensity. The second signal includes a voltage or current signal. A computer-processing module calculates the user's beat-to-beat heart rate from the second current signal.