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Light intensity detector

Gryd3

Jun 25, 2014
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R2 and the LDR will ALWAYS have the same potential across them if you are talking about them as a set.
The potential across one or the other will change.

So R2 (10K) and the LDR at 500Ω will let more current flow through them because the total resistance through them both is 10.5k. We can do the math and determine that:
9V / 10.5k = 0.8mA through both the LDR and R2
0.85mA * 10k = 8.5V potential across R2 only.
0.85mA * 500 = 0.5V potential across the LDR. (This happens to be the voltage at point B I was talking about)

Now if the LDR is in the dark, lets say they are both at 10k. The total resistance is 20k so we can determine:
9V / 20k = 0.45mA through both the LDR and R2.
0.45 * 10k = 4.5V across R2
0.45 * 10K = 4.5V across the LDR.

The resistance of the LDR changes which will change the current flowing through the LDR and R2... but using Ohms Law we can expect what voltage potential will exist across the LDR once we can figure out current and resistance.
You'll notice the two math examples, that even though more current flows through the LDR when it is bright, that is has a lower voltage drop... This may be hard to imagine or explain because voltage, current, and resistance changes at the same time.
Try to picture this from R2. The higher the current, the more potential will be across R2, which means that less will be across the LDR.


No tips for your presentation unless you want to experiment and get comfortable with what happens when you change R2. Maybe you can change it to stay off in normal light and turn on when you put your hand over it ;)
 

ayush soni

Aug 24, 2014
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dude its not 10K its 100K :p

& i think the total resistor is very high :( the light of let is very dim
100K + 330 Ω + LDR :p

& u know what
m F*cked :p
the exibition is at the day after tomorrow
& someone already submitted this project :p
anyways it was fun
i burned 5-6 LEDs :p lol :p
 

Gryd3

Jun 25, 2014
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& i think the total resistor is very high :( the light of let is very dim
100K + 330 Ω + LDR :p

The math will still apply, but there are not 3 resistors in series...
You have the LDR and one of the resistors in series, so those add up.
You have a single resistor in series with the LED so it should not be added to anything. (This value determines brightness)
 

Gryd3

Jun 25, 2014
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oh
yup u r right :D
Do you have a volt-meter or multi-meter that you can use to poke around your circuit with?
Study it. See what happens to the voltage at point B between the LDR and resistor. If you pay attention to the number here, you can adjust the value of the resistor to make the light turn on and off whenever you want.
 

Gryd3

Jun 25, 2014
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Haha, Good job on the project model.
Remember that you can swap out the resistor that you have connected to the LDR to something higher so you won't need to use the flash light anymore ;) Once you get the right value, you could either turn the lights out in the room, or activate it with the shadow from your hand.
I can't tell you what this value is though, as I don't have an LDR to test with right now and am unsure what it changes to in a normal lit room. (This is where the multi-meter helps.) You can either measure the voltage at point B and do some math to figure out what to replace the resistor to, or you could do it with trial and error and just start pluggin in bigger resistors till it works!

In any case, if you take some measurements let us know what values you get and we can help with the math and show you so you can do it again later without us.
 

ayush soni

Aug 24, 2014
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i submitted the model so dont have it now

anyways thxx for helping & supporting :)

electronics is very interesting thing :)

now what m thinking next is robots

wired control robot , wireless controller robot , linefollowing robot
etcc ;)


& i also want to make a thing i thing it is very tough

if u want i can send u a video
 

Gryd3

Jun 25, 2014
4,098
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The line following robots can be interesting.
You could do a simple robot, or a more complex one. The difference really is:
Make a track and the robot follows it, or make a maze and the robot solves it ;)

Robots can be quite the project too. You are going to need some background knowledge in electronics though if you will start dong this often.
Have you done any reading on Ohms Law or how transistors work yet?
 
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