Green Easy Pulse Sensor Heart Beat Sensor HRM2511E operates in transmission mode. The sensor body is built with flexible Silicone rubber material that helps to keep the sensor tightly hold to the finger. Inside the sensor case, an IR LED and a photodetector are placed on two opposite sides and are facing each other. When a fingertip is plugged into the sensor, it is illuminated by the IR light coming from the LED. The photodetector diode receives the transmitted light through the tissue on other side. More or less light is transmitted depending on the tissue blood volume. Consequently, the transmitted light intensity varies with the pulsing of the blood with heart beat. A plot for this variation against time is referred to be a photoplethysmographic or PPG signal.