Back to School with STEM-Limited-Time Event(Aug 6-Sep 5)
Story
Objective:
To design and build a basic laser-based security system that detects unauthorized entry using a laser beam and a photoresistor (LDR), triggering an alarm when the beam is interrupted.
Materials Required:
| Component | Quantity |
|---|---|
| Red Laser Diode Module | 1 |
| Light Dependent Resistor (LDR) | 1 |
| Buzzer or Piezo Alarm | 1 |
| Transistor (e.g., BC547) or MOSFET | 1 |
| Resistors (e.g., 10kΩ, 1kΩ) | 2–3 |
| Breadboard or PCB | 1 |
| 9V Battery or Power Supply | 1 |
| Wires and Connectors | As needed |
| Switch (optional) | 1 |
| Mirrors (optional for beam redirection) | As needed |
Working Principle:
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A laser diode emits a narrow beam of red light aligned directly onto the LDR (Light Dependent Resistor).
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The LDR’s resistance is low when illuminated by the laser, keeping the system in a "normal" state.
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If someone or something interrupts the beam (i.e., passes through it), the light on the LDR is blocked, causing its resistance to increase.
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This change is detected by a voltage divider circuit that triggers a transistor, which in turn activates a buzzer or alarm.
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The system resets once the laser beam is restored.
Circuit Description:
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The LDR and a resistor form a voltage divider. This output goes to the base of a transistor.
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Under normal conditions, the voltage at the base is insufficient to turn the transistor on.
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When the laser is interrupted, the voltage at the base increases, turning the transistor ON, and the buzzer is activated.
Optional Enhancements:
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Use a 555 Timer IC for better control.
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Add a relay to control larger alarms or lights.
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Use mirrors to direct the laser beam across wider areas (like doorways or windows).
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Include a microcontroller (like Arduino) to log events or send alerts.
Steps to Build:
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Connect the laser module and aim it directly at the LDR placed on the opposite end.
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Build the voltage divider circuit using the LDR and resistor.
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Connect the output to the base of the transistor.
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Connect the buzzer in the collector circuit of the transistor.
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Power the system using a 9V battery.
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Test the setup by interrupting the laser beam—the buzzer should sound.
Applications:
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Home or office security
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DIY educational projects
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Science fair or school electronics demos
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Entry detection in restricted areas
Precautions:
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Avoid direct eye contact with laser light.
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Ensure stable alignment between the laser and LDR.
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Use a proper resistor to protect the laser module from overcurrent.
