This LDR Light Sensor Circuit using BC547 works by itself.
When it is dark the LED turns ON.
When light comes then LED turns OFF.
LDR feels the light and controls the LED.
No need to press any switch.
Circuit is simple and is with very low cost.
It is good for small night lamp or garden light use.
Circuit Working:
Parts List:
| Component Name | Specification | Quantity |
|---|---|---|
| Resistors | 390Ω | 1 |
| LDR | 1 | |
| Multi Turn Preset 100k | 1 | |
| Semiconductors | Transistor BC547 | |
| LED any color 5mm 20mA | 1 | |
| Battery 9V DC | 1 |
In the above circuit diagram LDR changes resistance with light.
In day time, LDR resistance is low.
Then base voltage of transistor is low.
Transistor stays OFF and LED goes OFF.
In dark, LDR resistance becomes high.
Base voltage of transistor goes up.
Transistor turns ON.
Current flows and LED starts to glow.
Circuit Calculation:
Below is the formula for LDR Light Sensor Circuit using BC547.
Transistor BC547 base-emitter voltage (VBE) = 0.7V
LED voltage drop = 2V
LED current = 20mA
Supply voltage = 9V
Resistor R1 limits current through the LED.
R1 = (9V – 2V – 0.2V) / 0.02A = 340Ω (approx)
Nearest standard value = 390Ω
How to Build:
To build a LDR Light Sensor Circuit using BC547 follow the below steps:
- Take all the parts as shown in circuit diagram.
- Connect one end of the LDR to +9V.
- Connect the other end of the LDR to the base of BC547 through VR1 multi turn preset .
- Connect the emitter of BC547 to ground.
- Connect the LED anode to +9V through R1 390Ω.
- Connect the LED cathode to the collector of BC547.
Conclusion:
This LDR Light Sensor Circuit using BC547 is very simple and works automatic.
It feels light around it and turns LED ON or OFF.
Good small project for beginners to learn LDR and transistor working.