Could somebody please tell me what the name of the below circuit is please

Joshbessant2 · Apr 15, 2023

Hi all, does anybody know the name of this circuit and how it operates and the purpose of each component please? any help is much appreciated. Thanks in advance

DPG · Apr 15, 2023

It's a basic transistor switch circuit.

The light dependent resistor (lower left) forms a potential divider with resistor R3 and biases the transistor. At a certain point the voltage will rise to 0.7V (approx) and the transistor will switch on and illuminate the LED. R4 just limits the current to a safe value for the LED.

Joshbessant2 · Apr 15, 2023

DPG said:
It's a basic transistor switch circuit.

The light dependent resistor (lower left) forms a potential divider with resistor R3 and biases the transistor. At a certain point the voltage will rise to 0.7V (approx) and the transistor will switch on and illuminate the LED. R4 just limits the current to a safe value for the LED.

Thanks again! haha

DPG · Apr 15, 2023

Joshbessant2 said:
Thanks again! haha

No problem. It's good to have an understanding of basic building blocks like this. You can then break down more complex circuits and understand them better.

Joshbessant2 · Apr 15, 2023

DPG said:
No problem. It's good to have an understanding of basic building blocks like this. You can then break down more complex circuits and understand them better.

I agree, its currently what I'm trying to get my head around. Can I ask what the purpose of R2 is please?

DPG · Apr 15, 2023

Joshbessant2 said:
I agree, its currently what I'm trying to get my head around. Can I ask what the purpose of R2 is please?

In basic terms it limits the base current. Transistors (even bipolar ones like the BC108 here) only take a tiny amount of base current switch on.

Joshbessant2 · Apr 15, 2023

DPG said:
In basic terms it limits the base current. Transistors (even bipolar ones like the BC108 here) only take a tiny amount of base current switch on.

Thank you, you've been a big help!

Pastory · Jun 14, 2023

You know

DPG said:
It's a basic transistor switch circuit.

The light dependent resistor (lower left) forms a potential divider with resistor R3 and biases the transistor. At a certain point the voltage will rise to 0.7V (approx) and the transistor will switch on and illuminate the LED. R4 just limits the current to a safe value for the LED.

You know I have a little bit of a confussion about this circuit, can you show with arrows how current flows during day and during night

DPG · Jun 14, 2023

Pastory said:
You know

You know I have a little bit of a confussion about this circuit, can you show with arrows how current flows during day and during night

To be honest you'd be better off googling transistor switch circuits - there will be better explanations than I can type on a phone keypad. Having said that, if you still have any questions I'm sure we can help.