Fitness tracker and electrical telegraph: what they have in common

1. Step 1. Fitness tracker — has part — Pulse oximeter. An optical heart-rate sensor.
2. Step 2. Pulse oximeter — has part — LED. Red and infrared LEDs.
3. Step 3. LED — is part of — Traffic light. LEDs.
4. Step 4. Traffic light — has part — Relay. Relay controllers.
5. Step 5. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.

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