Automatic door and lead–acid battery: 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 — Smartwatch. A heart-rate and SpO₂ sensor.
5. Step 5. Smartwatch — has part — Lithium-ion battery. Powered by a lithium-ion battery.
6. Step 6. Lithium-ion battery — evolved from — Lead–acid battery. The lithium-ion battery is the next generation of rechargeable chemical current sources, with higher specific energy.

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