LED azul e optical telegraph: o que têm em comum

1. Passo 1. LED azul — é um tipo de — LED. A blue LED.
2. Passo 2. LED — faz parte de — Traffic light. LEDs.
3. Passo 3. Traffic light — evoluiu de — Railway semaphore. The first traffic light was a railway-style semaphore.
4. Passo 4. Railway semaphore — evoluiu de — Optical telegraph. The semaphore idea.

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