The module KY-039 uses ultra-bright infrared (IR) LEDs and phototransistors to detect the pulse of the finger. Because the light-emitting LED is on one side of the finger, and the phototransistor is on the other side of the finger, the phototransistor is used to obtain the emitted luminous flux. When the pulse passes through the finger, the resistance of the phototransistor will change slightly.
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