Gravity heart rate monitor sensor for Arduino is a thumb sized heart rate monitor designed for Arduino microcontrollers. This heart rate monitor sensor is a pulse sensor which is developed based on PPG techniques. This is a simple and low cost optical technique that can be used to detect blood volume changing in the microvascular bed of tissues. It is relatively easy to detect the pulsatile component of the cardiac cycle according to this theory. The sensor has two holes that you can use to attach to your belt. Users can wrap on their finger, wrist, earlobe or other areas where it has contact with your skin. A Gravity Interface is adapted to allow plug&play to ease the barrier of usage. The IO sensor shield is the best option to connect this heart rate sensor with your Arduino or other microcontrollers. Besides, this sensor can also be compatible with Raspberry Pi, intel edison, joule and curie by means of 3.3V input voltage.