Techniques systems and apparatus are described for implementing a heart rate sensor based on optical measurements of color response of human blood. An optical heart rate sensor can include light source circuitry including a first light emitting diode (LED) to output a first source light signal to a target measurement site a second LED to output a second source light signal to the target measurement site a first current pulse driver communicatively coupled to the first LED to modulate the first source light signal output from the first LED at a first predetermined frequency and a second current pulse driver communicatively coupled to the second LED to modulate the second source light signal output from the second LED at a second predetermined frequency different from the first predetermined frequency. The optical heart rate sensor can include detector circuitry including a light detecting photodiode connected to a bias voltage.
