Automatic Brightness Controller Using Ldr & Transistor

In the last post, we saw the working of LDR as a street light which was based on the concept of bjt as a switch. In this post, we will see LDR as a brightness controller

which is again based on the concept of BJT as a switch but the difference is that in the last project Our transistor was operating from the saturation region to the cutoff region. But the project we are going to discuss in this post acts totally in a reverse manner is in this project the transistor will operate from the cutoff region to the saturation region we will discuss the reason below for such operation.

I hope you all guys had done the project of LDR as a street light. So I'm not discussing the requirements and working of the components. I'm directly starting with the project.

# Here is the diagram of the project:-

In the last project, the LDR was connected between the ground and base terminal of the transistor.

In this project, we are connecting LDR between the positive terminal and the base terminal.

# Working of the project:-

Case:-1

When the torch(which is in on condition) is far away from the LDR at that time led is off because the LDR provides high resistance due to which the base current Ib is zero(Ib=0) and we know that if Ib=0 then Ic=0(since Ic=β*Ib) therefore transistors acts as an open switch and thus we can say that transistor is operating in cutoff region.

Case:-2

Now if we bring that torch near to the LDR then the resistance of LDR decreases and the base current which was zero becomes Ib=Ibmin due to which Ic=Icmin(since Ic=β*Ib) as result the led glows but its intensity is very much less. So in this case we can say that our q-point is tending to a saturation region or it is present some were near to the saturation region.

Case:-3

At the end, if we move the torch very much closer to the LDR, at this instance the resistance offered by the LDR is very much less and the LDR behaves like a normal wire therefore the base current changes from Ibmin to Ib=Ibmax due to which Ic becomes Ic=Icmax thus we can say that transistor is operating in saturation mode.

So as usual here is the video to boost your concept in a more detailed way.

VIDEO:-