Comment on a Circuit's Design

[Synesthetic]

"A student constructed the following circuit in order to switch on a security light when the light levels detected by a light dependent resistor dropped. He wasn't sure as to what light level would be the switching point. A voltmeter was added in order to check that the potential to the collect pin of the transistor was satisfactory. Comment on the design."

To be fully clear, the circuit elements from left to right, top to bottom, are:
[Cells (2), LDR, Resistor, Resistor, Voltmeter, Transistor (NPN), Security Light]

I'm evaluating solutions to some questions for a friend, but the problem is I didn't even do Physics this year so I'm a bit stuck!

And if it looks familiar - it's #35 from TSFX's Summer School Book 2.

Would appreciate any advice

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*Edit [3]* Defrosted Image / Reinforced Transistor Illustration / Blended Into Background

re. below: Thanks, Mao

[Mao]

I used to know this, and have a faint idea, but I'll have to read up on it again to be sure... tomorrow, hopefully

[cara.mel]

Hi,

I don't remember much of this either.
This is the half I am confident about (but so probably are you):
-When the light levels drop, the resistance of the LDR increases
-As the resistance of the LDR increases, the voltage across the LDR increases and hence the base voltage into the transistor increases (they're the same V)

This is the half that I am not sure about, I would seek another opinion:
-The rising base voltage will cause the transistor to become saturated
-This means the output voltage is 0V
-This means that basically nothing is getting to the light
-This means the light won't turn on

If I am correct, the circuit could be fixed by swapping the places of the LDR and the normal resistor below it

[Synesthetic]

This really helps, thanks a lot Mel. As you said I am also not sure of the transistor saturation point, but it does seem plausible!

After doing some light reading - another necessary change needed might be to shift the upper normal resistor down to the wire running horizontally directly below - to control the amount of current flowing to the base lead of the transistor thus protecting it from possible damage. (*edit* As I said though I'm not a Physics student so I'm not sure.)

It would be great if someone could confirm either of the above two points. But in any case I'm quite happy to have a suggested solution as of present

[cara.mel]

You'd be leaving the resistor up the top there I *think*. It has some usefulness to it.

By adding another one horizontal middle, yes it would decrease the volts getting to the transistor but this question does not have any numbers in it in the first place - perhaps the original arrangement is enough so that the transistor will never be in danger of blowing up (ie the battery has a very small V, or the second resistor is massive). In any case it would only be an improvement on the design and not a fixing of a problem present, which I suspect a problem is present or else we wouldn't be dealing with this question.

I used to know this stuff and it's really bugging me

[Synesthetic]

I will trust someone who got 49 in Physics over my ten minutes of reading over the textbook

Just attempting to solve this question has convinced me I would NOT make a good Physics student. To think I only switched from Physics to Literature in the second school week of this year...what a vital decision.

T-y once again!

[cara.mel]

After scanning through past exams in the hope of finding a similar question, I am fairly sure I am right. I am still not 100% so I would be happy to see someone else's opinion

However I have completely forgotten the purpose of half the circuit, such as the collector resistor

[kurrymuncher]

holy shit. do we have to know these kinds of things for physics in year 12??

[Synesthetic]

holy shit. do we have to know these kinds of things for physics in year 12??

Partly no - because I believe transistor (amplifiers) (?) have been removed from the 2009 course.