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A guide to LED Emergency Light Circuit with LDR

April 13, 2023 by Crystal Lin

In this project we will build a simple LED emergency light circuit using LDR.D Emergency Light Circuit with LDR .

What is LDR??

What is LDR (Light Dependent Resistor) as the name states is a special type of resistor that works on the photoconductivity principle means that resistance changes according to the intensity of light. Its resistance decreases with an increase in the intensity of light. It is often used as a light sensor, light meter, Automatic street light, and in areas where we need to have light sensitivity. It is also called a Light Sensor.

LDR are usually available in 5mm, 8mm, 12mm, and 25mm dimensions.

LED emergency lights energize when lighting is insufficient or mains power cuts. Earlier, fluorescent lights were used to build such circuits. But it turns out that the use of LEDs can provide adequate lighting for a longer period of time before the battery completely dies. In this project we will build a simple LED emergency light circuit using LDR.


Photoresistors or photoresistors are components that are sensitive to light. It is basically a photocell that works on the principle of light guiding. This passive component is basically a resistor whose resistance value decreases as the light intensity decreases. It's not uncommon for an LDR or photoresistor to have a resistance value of a few megaohms in the dark, then drop to a few hundred ohms in bright light.

 Hardware Components:

You will need the following parts to build this project.


1. Test Board

2. Cable

3. 12V DC

4. Resistor(33ohms)

5. Pot(50k)

6. Low-density lipoprotein

7. BD139npn

8. Bd140npn

9. 5mmLED


A Bipolar NPN transistor like BD139 is available in the SOT-32 plastic package. This kind of transistor is used in different electronic circuits because of less cost and high collector current. This transistor can drive upto 1.5A loads like motors, high power LEDs, relays, etc.


This circuit will turn on a 1 W LED when there is no light on the LDR face. Circuit operation depends on the change of LDR (Light Dependent Resistor) resistance. After completing the circuit, when the LDR is exposed to a sufficient amount of light, its resistance decreases. This then causes the voltage at the base of the PNP transistor to also drop.


When the LDR is exposed to a dark environment, its resistance increases, causing the voltage at the base of the PNP transistor to also increase. This voltage acts as a control signal for the BD139 NPN transistor, which triggers the output LED connected to a 33Ω 1W resistor. The control signal at the bottom of the PNP transistor can be tuned using a 50K potentiometer.


Used as an emergency backup for security systems and industrial sites.

It can be used as an emergency light in both home and office settings.

It can be used in study rooms and workplaces to avoid sudden power outages.


Crystal Lin

I like sharing my ideas and projects.

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