Story
Overview
This project demonstrates a simple light-sensitive switch: it turns lights OFF during the day and ON at night. Ideal for street lights, garden illumination, or home automation.
Materials Required
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LDR (Light Dependent Resistor / photoresistor)
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BC547 (or equivalent) NPN transistor
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Relay module (e.g. 12 V coil, SPDT)
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Resistors (e.g., 10 kΩ, 330 Ω)
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Zero PCB board or breadboard
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Power supply (e.g. 9–12 V DC)
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Diode (e.g. 1N4007) for back‑EMF protection
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Optional: manual switch, LED indicator
Circuit Diagram
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LDR forms half of a voltage divider, paired with a fixed resistor.
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The voltage divider’s output is fed to the transistor base.
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When it’s bright (day), the LDR resistance is low → base voltage low → transistor OFF → relay OFF → light OFF.
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In darkness (night), LDR resistance increases → base voltage rises → transistor switches ON, activating the relay and powering the light.
Step-by-Step Instructions
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Construct the voltage divider: LDR + fixed resistor → center node to transistor base through ~1 kΩ resistor.
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Use BC547 as a switch: emitter to ground, collector to relay coil.
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Relay coil powered from 12 V; diode placed antiparallel across coil.
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Connect the relay’s output contacts to drive the external lighting circuit or LED.
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Power the circuit with a steady DC supply.
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(Optional) Add a manual override switch in series with load or relay.
Calibration & Tips
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Adjust fixed resistor to set light/dark threshold; can use a potentiometer for tuning Arduino Forum.
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Add hysteresis or time delay (e.g. via averaging or capacitor) to avoid flickering from passing shadows or clouds.
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Install LDR away from the controlled light source to prevent feedback switching.
Example Arduino Variant
Some variations use an Arduino to read analog input from LDR and control a relay with code thresholds (e.g., activate when LDR reading drops below ~700)
Applications
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Street and outdoor lighting control
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Automated garden lights
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Emergency lighting systems
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Home automation for energy saving
Safety Notes
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When switching AC mains, take proper insulation precautions or use a solid‑state relay.
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For DC loads, ensure transistor and relay ratings are adequate.
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Always include a flyback diode across relay coils to protect transistor.
Troubleshooting
| Issue | Solution |
|---|---|
| Light stays ON in day | LDR may be shaded; increase sensitivity threshold or clean sensor |
| Lights flicker intermittently | Introduce time delay or hysteresis in design |
| Relay chatter/clicking often | Check power stability; use proper decoupling and delay |
| Transistor overheating | Ensure base resistor value correct; check transistor ratings |
