Electronics by Manmohan Pal: Simple LDR Circuit

Simple LDR Circuit

LDR Circuit : Automatic Street Light

Here we have explained a dark detector circuit by using 555 timer IC and a LDR (Light Dependent Resistor) which senses the light in surroundings and if it does not find the light, it triggers the IC and glows an LED attached with the circuit. In place of LED we can also use a buzzer or speaker to use it as dark detector alarm. The concept of this LDR circuit is quite simple and based on the working of LDR. So lets first understand more about LDRs.

Video link : LDR circuit using IC 555 Timer


LDRs are a type resistors made from semiconductor materials to enable them to have their light sensitive properties. There are many types of LDRs but one of the most popular material used is cadmium sulphide (CdS). These LDRs or also known as photo resistors works on the principle of “Photo Conductivity”. Now what this principle says is , whenever light falls on the surface of the LDR (in this case) the conductance of the element increases or in other words the resistance of the LDR falls when the light falls on the surface of the LDR. This property of the decrease in resistance for the LDR is achieved because it is a property of semiconductor material used on the surface.

Here in this dark detecting LED circuit, LDR is configured with 555 timer IC in ASTABLE mode in such a way that 555 ASTABLE generates square wave when the light intensity goes below a certain level.

1. pin no 6 is connected to pin no 2
2. pin no 4 is connected to pin no 8
3. a resistor of 5 k is connected between pin no 1 and pin no 2
4. a LDR is placed between pin no 4 and pin no 2
5. LED is connected between pin no 1 and pin no 3

When there is dark on LDR the LEDs are ON  i.e. at night lights will glow automatically
Where there is light on LDR the LEDs are off  i.e. in day time lights are off automatically


above figure shows the circuit diagram of dark detecting LED. This circuit shows how we can convert a simple astable mode circuit to a “dark sensor”. We just need to add an LDR and a resistor in the circuit, then it simply works as dark detector. A general purpose LDR is used for sensing the light. When proper light is falling on the LDR its resistance is very low. When there is no light the LDR resistance increases. At this time the IC is triggered and glows the LED attached with the circuit

 Automatic Street Light, using ic 555 timer, by Manmohan Pal

In this project, we are going to make a Simple Automatic Street Light Controller Using Relay and LDR. This circuit is very simple circuit and can be built with LDR, and 555 IC to trigger the AC load with relay. Here we have used an AC bulb as street light. Some applications of this circuit are street light controlling, home/office light controlling, day and night indicators, etc.

Components Required:

1) IC 555 Timer
2) LDR (Light Dependent Resistor)
3) Relay 12 volt
4) Resistor 10k or  20 k variable resistor
5) Power Supply 12v -1
6) Connecting wires
7) AC supply
8) AC Load or Bulb

Manmohan Pal
Mob. 8989811397
Email- mannmohanpal@gmail.com
Blog: http://electronics4ubymanmohanpal.blogspot.in/p/blog-page_5.html
Youtube Channel: https://www.youtube.com/channel/UCDnhARnHOEIPuNIEp5vPdGQ

Video: https://www.youtube.com/watch?v=ub1yagSXBlY

Alternate blinking LED- Police Car Lights Using Ic 555 Timer in Astable mode -by Manmohan Pal


  1. Replies
    1. Please make an easy circuit diagram for the Apprentice like me !that's why I can make it follow your circuit! A lot of thanks for your video as well as all of your upcoming videos.

  2. 4 channel module me components kya kya lagega

  3. sir rf Transmitter rf Receiver kya app bana sjte ho

  4. Please write the codes of the projects in your blog

  5. Sir how to download code for clock

  6. Sir show to download code for clock and remote realy

  7. i need circuit diagram of Arduino board..