Automatic door and fitness tracker: what they have in common

1. Step 1. Automatic door — has part — Motion sensor. A sensor.
2. Step 2. Motion sensor — uses — Infrared radiation. Passive infrared.
3. Step 3. Infrared radiation — is used in — Pulse oximeter. Infrared absorption by haemoglobin.
4. Step 4. Pulse oximeter — is part of — Fitness tracker. An optical heart-rate sensor.

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