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Rojony Khatun
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Last updated: September 21, 2025

LDR Light Sensor Circuit Project Using BC547 Transistor

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BC547 Transistor

BC547 Transistor

General-purpose NPN bipolar junction transistor (BJT)

LDR (light-dependent resistor)

LDR (light-dependent resistor)

A resistor whose resistance decreases with increasing incident light intensity; also called a photoresistor

3V LED 5mm

3V LED 5mm

3V, 5mm round light-emitting diode

4.7k Fixed Resistor 1/4W

4.7k Fixed Resistor 1/4W

A 4700-ohm resistor rated for 1/4 watts

100R Fixed Resistor 1/4W

100R Fixed Resistor 1/4W

A 100-ohm resistor rated for 0.25 watts

*Please note: These are affiliate links. I may make a commission if you buy the components through these links. I would appreciate your support in this way!

LDR Project

Build an LDR-based light sensor circuit using BC547. Detect ambient light levels to control LEDs or relays automatically for DIY electronics projects.

LDR BC547 light sensor circuit

One of the applications of the LDR project is flipping lights, such as LEDs or relay,s based on the amount of light present in the environment with the help of a BC547. It is perfect to study the idea of transistor switching and to creathomemadede light-sensitive circuits.

simple light detection project

LDR Project Using BC547 is a simple electronic circuit to measure the amount of light present in the environment and use it to power a load automatically, in this case, an LED or a small relay. The LDR is a type of resistor that varies its resistive value with the intensity of light. BC547 is an NPN transistor that is used as a switch to switch the load ON or OFF depending on the voltage on the LDR. At a certain point of light level, the voltage on the LDR activates the BC547 transistor, and the device connected to it is operated. In bright light, on the contrary, the transistor becomes non-conducting, putting the load OFF. The students, hobbyists, and beginners who learn about the light sensors, transistor switching, and automatic control circuits would find this project to be ideal. Components are cheap and can be fitted on a PCB or breadboard. The series resistor is adjusted with the LDR, so that you can adjust the light sensitivity to what you want. They can be used as automatic night lamps, garden lights, or simple lights to trigger an alarm.

Work / Installation (Inputs β†’ Outputs)

  • Power Input β†’ DC supply (5–12V).
  • LDR Sensor β†’ Senses ambient light levels.
  • Voltage Divider β†’ LDR and resistor set threshold voltage.
  • BC547 Transistor β†’ Switches load ON/OFF based on sensor voltage.
  • Load Output β†’ LED, relay, or small device controlled automatically.
  • Installation β†’ Assemble circuit on PCB/breadboard; connect LDR, load, and DC power; adjust resistor for desired light sensitivity.

Testing & Final Adjustments

Once assembled, supply DC and monitor the LED or load behavior under different light conditions. Darkness (cover the LDR): the BC547 is expected to conduct and to put the load ON. Let it be exposed to bright light, and the transistor must cease to conductt and the load should be turned off. The variable series resistor to adjust sensitivity. Check all connections to ensure that they are stable and well-oriented transistors. Several test cycles provide a good deal of switching reliability and consistency. Correct testing will guarantee that the LDR circuit has been successful in achieving automatic night lamps, garden lights, or alarm-activated lights.

Frequently Asked Questions - LDR Project:

What is LDR?

Light Dependent Resistor whose resistance varies with light intensity.

Why use BC547?

It acts as a switch to control LEDs or other devices based on LDR voltage.

Can it control relays?

Yes, with proper relay rating compatible with BC547 current.

What voltage is required?

Typically 5–12V DC supply.

Is it beginner-friendly?

Yes, simple components and easy to assemble on breadboard or PCB.

Can I adjust sensitivity?

Yes, by changing the series resistor value with LDR.

Can it be used for night lamps?

Yes, ideal for automatic night lamps and garden lights.

Do I need additional protection?

Optional fuse or diode for load protection.

What type of LED can be used?

Standard low-current LEDs or small 12V LEDs with resistor.

Is it cost-effective?

Yes, uses inexpensive components like BC547, resistors, and LDR.

Rojony Khatun

Rojony Khatun

CalculatorToolsProject

Rojony Khatun is the creator of a free online tools website designed to simplify everyday and engineering-related calculations for students, professionals, and hobbyists. The platform offers a wide variety of basic and advanced calculators across key categories, including Electrical Calculators, Electronic Calculators, and Time & Date Calculators, helping users solve problems quickly and accurately.