KY-039 Heartbeat Sensor Module

This module consist of bright infrared (IR) LED and a phototransistor to detect the pulse of the finger, a red LED flashes with each pulse.
The resistance of phototransistor will change slightly in correlation to blood pressure, as more light will be absorbed during the buildup of blood in vessels.
You need to stick your finger between LED and phototransistor to get reading.
Obviously ;-)