Abstract: According to various aspects of the present disclosure, an Optical Heart Rate Sensor is designed to pick up the heart rate pulse when worn on a user's wrist instead of wearing a bulky chest strap, which is not comfortable. An example optical heart rate sensor unit has two light emitting diodes that provide light source projected to skin. An optical detector mounted close to the light source can detect the movement of blood under the skin of the wrist based on light reflected from the skin. The optical detector can detects blood movement by emitting light onto skin and measuring the amount of light absorbed by skin. The optical sensor can be very sensitive to light, electrical static and electromagnetic waves. A special chamber is designed to protect the sensor unit so that the sensor unit can work when user wears the sensor on the arm.