AUTOMATIC STREET LIGHT SYSTEM

1. Aim

To design and demonstrate an automatic street light control system using an LDR (Light Dependent Resistor) and a two-transistor switching circuit.

2. Components Used

S.N

Component

Function

1

LDR (Light Dependent Resistor)

Senses environmental light level

2

Q1 – NPN Transistor

Acts as day/night sensor switch

3

Q2 – NPN Transistor

Drives the LEDs (load switching)

4

R1 – 33kΩ Resistor

Load output resistor for Q1 to prevent short circuit & limit current

5

R2 – 1kΩ Resistor

Base resistor for Q2 to limit trigger current

6

Potentiometer – 100kΩ

Adjusts sensitivity/trigger threshold for Q1

7

3.7V Lithium-ion Battery

Power supply (recommended due to lack of LED current limiting resistor)

8

LEDs

Output indicator (street light)

3. Circuit Diagram (Actual Explanation)

Block Description:

LDR → Voltage Divider → Q1 (Sensor Transistor) → Q2 (Driver Transistor) → LED Load

 Actual Circuit Behavior:

  • At day, LDR = Low resistance → activates Q1 → deactivates Q2 → LEDs OFF
  • At night, LDR = High resistance → Q1 OFF → Q2 ON → LEDs glow

4. Working of the Circuit

 Day Condition

  • Sunlight falls on the LDR → its resistance becomes low (kΩ).
  • LDR + Potentiometer form a voltage divider.
  • Sufficient voltage (~0.7V) appears across base–emitter of Q1, so:

✔ Q1 turns ON
✔ Q1 pulls its collector (connected to R1) to ground
✔ Base of Q2 gets no trigger
✔ Q2 remains OFF
✔ LEDs turn OFF

Thus, during the day, the light stays OFF automatically.

 Night Condition

  • No light on the LDR → its resistance becomes very high (Mega ohms).
  • The voltage divider cannot produce required 0.7V at Q1 base, so:

✔ Q1 stays OFF
✔ R1 (33k) pulls Q1 collector UP
✔ Q2 base gets a HIGH signal through R1 & R2
✔ Q2 turns ON
✔ LEDs receive power
✔ LEDs light ON

Thus, during night, the street light turns ON automatically.

5. Importance of Each Component

✔ R1 – 33kΩ

  • Works as load output resistor for Q1
  • Prevents short circuit
  • Limits leakage current
  • Pulls signal HIGH when Q1 is OFF

✔ R2 – 1kΩ

  • Current limiting resistor for Q2 base
  • Ensures proper triggering without damaging the transistor

✔ 100kΩ Potentiometer

  • Used for sensitivity adjustment
  • Controls at what light level Q1 should turn ON/OFF
  • Helps set day/night threshold manually

✔ Q1 (Sensor Transistor)

  • Detects voltage from the LDR divider
  • Turns ON during day, OFF during night

✔ Q2 (Driver Transistor)

  • Default ON due to pull-up path
  • Turns ON LEDs at night
  • Turns OFF LEDs during day

✔ LDR

  • Main light sensor
  • High resistance = night
  • Low resistance = day

✔ 3.7V Lithium-Ion Battery

  • Recommended because:
    • No LED protection resistor present
    • Higher voltages (5V/12V) may damage LED load
    • Stable discharge & lightweight

6. Conclusion

“At night, LDR provides high resistance → Q1 remains OFF → R1 pulls signal high → Q2 turns ON → LEDs turn ON.
At day, LDR provides low resistance → Q1 turns ON → Q1 grounds R1 → Q2 turns OFF → LEDs turn OFF.”

The circuit successfully performs automatic street light control using two transistors and an LDR.

7. Applications

  • Street lighting
  • Garden or pathway lights
  • Home night lamp automation
  • Solar street light automatic ON/OFF

8. Advantages

  • No microcontroller required
  • Low power consumption
  • Simple transistor-based design
  • Automatically responds to environment

9. Precautions

  • Do not use battery above 3.7V unless LED resistor is added
  • Protect LDR from rain/dust for long life
  • Ensure correct polarity of transistors
  • Properly set potentiometer for good sensitivity

 

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