KY-039 is a basic sensor module that measures heart rate by the principle of infrared light transmission (photoplethysmography / PPG) by placing the fingertip between two components. The module has an infrared LED and a phototransistor placed opposite each other; when the finger is placed between the two, the change in blood flow with each heartbeat changes the transmittance of the infrared light and this signal is reflected on the analog output. Since the Pulse Sensor uses infrared transmission as opposed to green light reflection, the finger must be held steady in the center of the sensor for measurement. With its 3-pin structure (VCC, GND, S), it can be used directly with the analog pin of Arduino and ESP series boards. It is much more cost-effective than Pulse Sensor and is preferred for basic heart rate detection and educational biometric projects.
