Light pen and tunnelling shield: what they have in common

1. Step 1. Light pen — has part — Photodiode. A photocell catches the CRT beam.
2. Step 2. Photodiode — is part of — Pulse oximeter. A photodiode.
3. Step 3. Pulse oximeter — has part — LED. Red and infrared LEDs.
4. Step 4. LED — is part of — Street light. LEDs.
5. Step 5. Street light — is part of — Tunnel. Lighting.
6. Step 6. Tunnel — uses — Tunnelling shield. Shield tunnelling.

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