Experiment with Light Dependent Resistor Circuit

Light dependent resistor or LDR is another component that you should learn about if you are new to electronics project design. This component has a resistance value which vary with the intensity of light that shines upon it.



The typical value of LDR is greater than 1 Mega ohm if it is located in a dark environment. This value reduced to less than 1,000 ohm when light falls upon it. 

This component is made of Cadmium Sulphide or Cds and you can recognized it by the spiraling track on the top surface of the component. Test the functionality of this part by using a multimeter that measures the resistance of a component.

First, place the component on a table and cover it with a non transparent paper. Measure the resistance. Do the same by shining a bright light onto its surface.


A LDR mounted on a breadboard.


Light Dependent Resistor Application Circuit 


As shown in the circuit, the core component is operational amplifier LM358 and light dependent resistor. LM358 is configured with a hysteresis formed by it and the resistor R3 of 47K. This prevent the chattering of relay when the light intensity is unstable.

Two resistors R1 and R2 are used as potential divider that sets the reference voltage at positive input of the op-amp at 6V(when a 12V input is used).

The negative input of the op-amp is formed by variable resistor VR1 and the LDR. When the LDR is in the dark, the resistance will be very much higher than the VR1 causing the negative input to be higher than the positive input of the op-amp.

As a result, the output from LM358 will be LOW causing transistor Q1 to turn ON. The relay will also be ON. You can connect LIVE to the COM of the relay and a LAMP to the NO and Neutral to the end of the LAMP. This will cause the LAMP to turn ON in a dark environment. 

In a bright environment, the LDR will have a resistance of about 1K causing the input voltage at the negative terminal to be close to 0V. This caused the output of the op-amp to be HIGH resulting in the switching OFF of PNP transistor Q1. This in effect turn OFF the relay and the LAMP will switch OFF.

One application of this circuit is to use it as a night light along the walkway. In this way, the light automatically lights up when night comes.


LDR Application Parts List


R1, R2 22K ohm 5% 1/4W Carbon Film Resistor
R3 47K ohm 5% 1/4W Carbon Film Resistor
R4 2.7K ohm 5% 1/4W Carbon Film Resistor
R5 1.2K ohm 5% 1/4W Carbon Film Resistor
VR1 100K ohm Potentiometer
LDR Light Dependent Resistor
Q1 2SA1015 PNP Transistor
D1 1N4003 Diode
U1A LM358 OP-AMP
L1 AC LAMP




Back to Light Dependent Resistor Home Page



Recent Articles

  1. Electronics Design Contest

    oin electronics design contest and win prizes. Test your hardware and software skills against the best designers from the rest of the world.

    Read More

  2. Electronics Events

    Join the electronics events to enhance your knowledge and network with other professionals in this industry.

    Read More

  3. Engineering Free Magazine

    Some engineering free magazine available for you to subscribe today

    Read More

New! Comments

Have your say about what you just read! Leave us a comment in the box below.


Digital Dice 
Project

Explore the use of 7-Segment Display, 555 Timer, Decade Counter and Binary Adder. Get the circuit.

Battery Tester 
Project

Design and build a battery tester to test dry cell and rechargeable battery with a voltage of less than 2V. Check here


Door Bell

Construct this simple door bell chime and have fun. Find out more here



Electronic Kits

If you are looking for electronic kits, electronic gadgets or robotic kits, get them here


Home Alarm 
Project

Build this simple home alarm to protect your house from intruders. See the schematic circuit