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Automatic Street Light Controller Using Ldr & Transistor

In the last post, we studied BJT as a switch. If you want to revise that concept then just watch the video given below.
For those who had not read the concept of bjt as a switch just read it and then come on to this post so that you can understand this project with great depth because we are making this project by applying the concept of BJT as a switch.
Now as I promised you that after completing 70%-80% of the transistor model I will be explaining to you some projects. So we will be doing a total of 3-4 projects this is the 1st project. I had also provided the PCB layout of this project at the end of this post because I want you all to do this project on the PCB rather than implementing all the components on the breadboard.
So starting with our project. First of all, we should understand the components that we are using in this project and its working.
Components used are:-
Resistors (50k, 500)
Battery (9v) 

Working of the project:-
Here is the diagram of our project.
So when our LDR is suffering from the darkness that is when the torch(which is in on condition) is far away from the LDR at that time ldr offers high resistance to the circuit therefore current Ibmax moves towards the base terminal of the transistor and almost negligible current flows from the branch of the Ldr. As Ibmax flows to the base terminal therefore current flowing to the collector terminal is also maximum (Icmax) since Ic =β*Ib due to which the output is high that is led remains on thus we can say that the transistor operates in the saturation region.
Now if we move that torch near to the LDR, at this instance ldr operates as a normal resistor due to which some current flows to the base terminal of the transistor and some to the LDR branch due to which Ibmax becomes Ibmin and Icmax becomes Icmin and the led becomes little dim.
At the end if we bring that torch very much closer to the LDR then LDR offers negligible resistance and therefore maximum current flows from the branch of the LDR and the current flowing through the base terminal becomes zero(i.e. Ib=0) due to which Ic flowing through the collector terminal also becomes zero (i.e. Ic=0) since Ic =β*Ib thus we can say that our transistor acts in cutoff region.
Here is the video of my explanation your concept would be more clear on watching this video.

So as I promised, here is the pdf of Pcb layout of this project 

Photo of PCB layout:-
I had given this photo so that it will be easy for you to understand the placement of components on the board.

If you understand this concept then please let me know in the comment section given below.


  1. Thanks for taking the time to share this informative article here. Looking for the Led Sports Lights then is the place which you are looking for.

    1. Welcome, please do follow my posts I'm trying to explain all the concepts in great depth.

  2. This article has truly inspired me. The way you explained each point makes it easy to follow and implement. Thank you for the motivation! semiconductor wafer suppliers


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