A street light works by using a photocell to detect ambient light levels, automatically turning the light on at dusk and off at dawn.
Here's a breakdown of the components and process:
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Photocell (Light Sensor): This is the key component that senses the amount of light. It's a resistor whose resistance changes based on the light falling on it.
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Circuitry (Transistor & Relay): The photocell is connected to a circuit containing a transistor and a relay.
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Daytime Operation: When light shines on the photocell, its resistance decreases. This allows a small current to flow, turning the transistor on. The transistor then energizes the relay's electromagnet. The electromagnet pulls a switch open, which disconnects the power supply to the street light, turning it off.
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Nighttime Operation: When it gets dark, the photocell's resistance increases dramatically. This prevents current from flowing through the transistor, effectively turning it off. Consequently, the relay's electromagnet is de-energized. A spring then pulls the switch closed, connecting the power supply and turning the street light on.
| Component | Daytime (Light) | Nighttime (Dark) |
|---|---|---|
| Photocell | Low Resistance | High Resistance |
| Transistor | On (Activated) | Off (Deactivated) |
| Relay Electromagnet | Energized | De-energized |
| Switch | Open (Light Off) | Closed (Light On) |
In essence, the street light utilizes a simple, automated system to respond to changes in ambient light, providing illumination when needed and conserving energy during daylight hours.
