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Rojony Khatun
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MaΕ‚gorzata Koperska, MD Steven Wooding
Last updated: September 21, 2025

Light Sensitive Automatic On-Off Light Using LDR Sensor

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100W Light

100W Light

High-intensity electric bulb or LED unit

100K Fixed Resistor 1/4W

100K Fixed Resistor 1/4W

A 100,000-ohm resistor rated for 0.25 watts

[100k-fixed-resistor-1/4w]
DB3 DIAC

DB3 DIAC

Bidirectional trigger diode with a breakover voltage of ~30V

10K Fixed Resistor

10K Variable Resistor

Adjustable resistor with 10,000 ohm resistance

LDR (light-dependent resistor)

LDR (light-dependent resistor)

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

BT136 Triac

BT136 Triac

A bidirectional triode thyristor (TRIAC) commonly used for switching and phase control of AC loads up to 4A.

2-pin Plug

2-pin Plug

Basic electrical plug with two metal pins

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Light Sensitive Automatic On Off Light LDR

Automatically switch lights ON/OFF using the LDR sensor. Detect ambient light levels to control LEDs or small lamps for energy-saving DIY projects.

DIY light ON-OFF project

An LDR light-sensitive automatic light ON- OFF light switches lights on in the dark and off in bright lights. It shows the detection of ambient light and a transistor-based switching of DIY electronics projects.

energy-saving light switch

The Light Sensitive Automatic On-Off Light Using LDR is a viable electronic project that turns a light ON when there is no light and OFF when it is bright. The LDR (Light Dependent Resistor) is a resistor that varies the resistance based on the light around. A switch is given by a transistor such as BC547. The transistor will conduct when the intensity of light is below a level, causing the relevant LED or lamp to turn on. Under bright conditions the transistor is not conducting and this switches the light to the OFF position. The project will be suited to students, hobbyists and DIYers who will study the concepts of light-sensitive circuits and automatic switching. It includes cheap and readily available materials. Sensitivity fine-tuning is achieved by adjusting the series resistor with the LDR. They can be used in automatic night lamps, garden lights, and the basic energy saving lighting systems.

Work / Installation (Inputs β†’ Outputs)

  • Power Input β†’ 5–12V DC supply.
  • LDR Sensor β†’ Detects ambient light levels.
  • Voltage Divider β†’ Sets threshold voltage for transistor switching.
  • BC547 Transistor β†’ Switches light ON/OFF based on LDR signal.
  • Load Output β†’ LED, lamp, or small light controlled automatically.
  • Installation β†’ Assemble on PCB or breadboard; connect LDR, load, and DC supply; adjust resistor to set sensitivity; verify transistor orientation.

Testing & Final Adjustments

Once assembled, impose DC power and cover the LDR to represent darkness. The lamp or LED connected to it must go on. Bright light be exposed to the LDR; the load should be turned off. The series resistor can be adjusted to make fine tuning. Make sure that there is proper polarity of transistors and no loose connections. Repeat operation to ensure that it is working. An effective test will ensure that automatic switching can be used with confidence and night lamps, garden lights or energy saving lighting which is a simple and practical introduction to light-sensitive electronic circuits.

Frequently Asked Questions - Light Sensitive Automatic On Off Light LDR:

What is LDR?

Light Dependent Resistor whose resistance changes with light intensity.

Why use BC547?

It acts as a switch to control lights based on LDR signal.

What voltage is required?

Typically 5–12V DC supply.

Can it control a lamp?

Yes, within transistor current rating.

Is it beginner-friendly?

Yes, simple and inexpensive project.

Can sensitivity be adjusted?

Yes, by changing the series resistor with LDR.

Can it be used for night lamps?

Yes, ideal for automatic night lamps and garden lights.

Do I need additional protection?

Optional diode or fuse for load safety.

Can multiple LEDs be used?

Yes, within transistor current limits.

Is it energy-saving?

Yes, lights turn off automatically in bright conditions.

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.