? simple heart-beat transducer can be made from an infrared LED and an infrared phototransistor. ?t works because skin acts as a ref1ecti?e surface for infrared light. The lR ref1ecti?ity of one's skin depends ??? the density of blood in it. Blood density rises and falls with the pumping action of the heart.
80 the intensity of infrared ref1ected by the skin (and thus transmitted to the phototransistor) rises and falls with each heartbeat.