# LIGHT SENSOR USING LDR AND TRANSISTOR...need help asap.

#### Mihir1998

Joined Jan 24, 2016
7
I've made a circuit as attached and it is working. No problems with that...but I'm unable to understand the exact flow of current and electons and the role of each component in the circuit...please help me out...i need to know everything before presentation which is on 28th Jan!!

#### atferrari

Joined Jan 6, 2004
4,785
Do not be lazy Mihir; put a multimeter in series with every part of the circuit and measure current while noting the polarity.

Make sure of two things:

that you keep the polarity of your probes consistent for all the measurements,
that you test the circuit in two well defined conditions of that LDR.

That could take few minutes and you could have a few hours of good sleep before the 28th, I guess.

#### Mihir1998

Joined Jan 24, 2016
7
But I want to know the purpose of all the components....i do have a rough idea bt need some details...and also the working of transistor...

#### atferrari

Joined Jan 6, 2004
4,785
Measure all currents first and note polarities. That you can do alone.

#### Mihir1998

Joined Jan 24, 2016
7
Alrite...den??

#### atferrari

Joined Jan 6, 2004
4,785
Alrite...den??
No idea what you are saying here.

I give up. Buena suerte.

#### Jony130

Joined Feb 17, 2009
5,528
The BJT in this circuit acting as "switch". 330 ohms resistor limits (sets) the LED current. 220k ohms resistor together with LDR form a light dependent voltage divider. The resistance of a LDR decreases when the light intensity increases. So the output of the voltage divider decreases when the light intensity increases. And this will tur-off the BJT.

#### Mihir1998

Joined Jan 24, 2016
7
The BJT in this circuit acting as "switch". 330 ohms resistor limits (sets) the LED current. 220k ohms resistor together with LDR form a light dependent voltage divider. The resistance of a LDR decreases when the light intensity increases. So the output of the voltage divider decreases when the light intensity increases. And this will tur-off the BJT.
Thank u so very much jony!!!

#### Alec_t

Joined Sep 17, 2013
14,384
.i do have a rough idea
Tell us what the ideas are, then we can help further (but not do your homework for you).

#### Mihir1998

Joined Jan 24, 2016
7
Its not my homework...it's my project...

#### bertus

Joined Apr 5, 2008
22,290
Hello,

You will learn more by examening the ciruit and find out yourself how it works.
That way you can make your presentation more successfull.
If there are any questions during the presention and you got the explanation from someone else,
how would you react on the questions.

Bertus

#### Mihir1998

Joined Jan 24, 2016
7
Hello,

You will learn more by examening the ciruit and find out yourself how it works.
That way you can make your presentation more successfull.
If there are any questions during the presention and you got the explanation from someone else,
how would you react on the questions.

Bertus
Alrite...thank you bertus....I'll try tomorrow...if I still have some doubts then I will ask!!

#### Mihir1998

Joined Jan 24, 2016
7
Tell us what the ideas are, then we can help further (but not do your homework for you).
My project is use of an ldr for automatic light control...these are some pics...now I just need a clear idea of how the circuit works...and what exactly changes occur when the ldr is covered so that the led gets switched on!

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