LDR Circuit

Introduction: LDR Circuit

About: An engineering student who loves music and innovating.


Homes are getting smarter each day because of the technology surrounding currently. An application used in these smart homes is the LDR system. This article will show you how to construct your own LDR system with simple tools and how to test it using computer soft wares and designing.

Step 1: Introduction

Light is the key of everything, you cannot see without light reflection. Sometimes people need everything to be quicker and smarter, maybe due to busy or lazy hours. Thus, this circuit has an important role in our lives. By using the LDR, you can control an adjust every single detail related to light in your room for example. LDR is used here to allow the current to go through the LED to turn on when needed. You can adjust it to turn on during day when it senses light, or during night when it senses darkness. Thinking that way will make it easier for you to turn the lights on when needed without leaving your place using a transistor as a gate or switch to allow the current by the sense received from the LDR with the help of some resistors to limit the current for a longer life-time. This can be taken to another level and control everything depending on light and photosensors to be smarter and mor electronic than the traditional ways.

Step 2: What Is LDR

It’s a semiconductor photoresistor called that senses the light and darkness to give corresponding resistance to the circuit depending on how much light it senses. The relation between the resistance and light is inversely proportional, as the lighter it gets, the more conductive it becomes, and of course produces less resistance due to increasing its conductivity

Step 3: List of Components

To design this circuit, of course you need to draw the schematic first. Those are the components needed to draw the schematic:

1- 10k ohm resistor.

2- 360ohm resistor.

3- LDR.

4- LED.

5- BC 547 transistor.

9V battery.

Step 4: Schematic

By using KICAD software, we can draw the schematic circuit with values annotated as shown in figure (1).

Step 5: Connection

A simple software is used to connect this circuit visually, which is the Tinker Cad. As shown in figure (2), the circuit is connected using the components listed above.

Here is how to connect this circuit in a few simple steps:

1- Place a bread board to help you in connecting.

2- Connect the 9V battery terminals to the positive and negative rails respectively.

3- Connect the 10k resistor to the positive rail, and the other terminal of it to an open rail.

4- Connect the LDR to the same rail, and the other terminal to the negative rail of the breadboard.

5- Place the transistor into 3 different rails, as each rail in breadboard represents 1 point.

6- Connect the emitter terminal to the negative rail of the board.

7- Connect the base terminal to the same rail that connecting the 10k resistor and LDR just between them.

8- Connect the collector terminal to the cathode terminal of the LED.

9- Connect the positive terminal of the led to a terminal of the remaining 360 ohms resistor.

10- Connect the remaining terminal of the resistor to the positive rail of the board.

11- Make sure that you connect the positive and negative rails of the board from both sides to each other to maintain the closed loop so that the circuit can work.

Step 6: Simulation

By using the same software (Tinker Cad), you can see whether the circuit is working properly or there’s some mistakes or errors that you need to take care of like changing the values or checking the LDR’s sensitivity. As shown in figure (3) and (4), the circuit is working properly with these values. In figure (3), the LED is off, when the LDR senses much light.

In figure (4), the LED is on as the LDR senses the darkness.

Step 7: Creating the PCB

With KICAD that we used in the schematic; we can get a PCB to be drilled later by a fabrication manufacturer. In figure (5) the final shape of PCB.

Some hints you need to take care of while designing a PCB:

1- Make sure that the holes of the footprints are located exactly on the grid points.

2- Make sure the you select the proper layer.

3- Make sure of the trace width,

4- Make sure of having all components close to each other to not waste material and money.

5- Make sure that you added proper edges and filled zoned to maintain the copper layers.

Step 8: 3D Views of PCB.

Here you can see front and back 3D views of PCB.

Step 9: Conclusion

It is brilliant to use technology to do your traditional doings. This will give you inspiration to make the world easier not just your home, just consider some principals to be always taken into account. First principal is that time matters, and that’s the key of getting everything smarter and quicker, just by making the best use of both technology and environment.

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