# Light intensity detector

Discussion in 'LEDs and Optoelectronics' started by ayush soni, Aug 24, 2014.

1. ### ayush soni

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Aug 24, 2014
i want to make a light intensity detector which will turn on the LED when the intensity of light is low & turn LED off when the intensity of light is high

& if u didnt understand let me know i will explain in more detail
& i want circuit as simple as possible
bcoz m a newbie

2. ### Gryd3

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Jun 25, 2014
you could google how a nightlight works
Should be a ton of examples

3. ### ayush soni

92
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Aug 24, 2014
but the problem is m newbie & want a simple curcuit

4. ### Gryd3

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Jun 25, 2014

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Dec 18, 2013
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6. ### Gryd3

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Jun 25, 2014
I've got another circuit drawn up that will be more robust, and suitable for a final project or design.
The first link showed a very very simple method, but it is only useful in the context that you use a 3V button cell battery and don't plan for it to last very long.

With a couple more parts you can build this. It can also be adjusted to use with different voltage sources.
The example will drive an LED, but you can just as easily make it drive a bigger load.
If you would like your circuit to be adjustable, you can lower the value of R2, and put put trim-pot or potentiometer in series with it. (Keeping a resistor there will prevent you from burning out your photocell if a very little resistance is present at R2)

Otherwise, the values used can be loosely chosen.
R2 : 300 - 430Ω (Lower values will require a darker room to trigger, this value may be changed depending on the photo-cell being used.)
R3 : 1kΩ or similar (The value here does not matter so much, as Q1 will be conducting when in a light room, so a large value is chosen to prevent excessive current draw.)
R4 : This is chosen based on what YOU want to drive. and what YOU want to power the circuit with. (6V supply, and 220Ω will provide you with 20-30mA for an LED depending on LED color)

You can grab some NPN transistors from radioshack or similar, just be mindful not to try to drive a really heavy load like a motor with them until you can look at the specs on em to determine how much current they can handle.

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Dec 18, 2013
Good Jerid
what are NPN resistors?

8. ### Gryd3

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Jun 25, 2014
New technology... I think Typo Industries makes them.
Good catch. I've corrected the above post.

9. ### ayush soni

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Aug 24, 2014
thxx @Gryd3 & Arouse1973
& i found 1 more simple circuit

in which a 9V battery is connected with 1K ohm ressistor & then it is connected with LDR & then LED & again to battery

but it work is opposite

when the led get turn of when the intensity is high & turn of when intensity in low i want its opposite (turn on led when intensity is low & tuen it of when its high)
& since i am a newbie ihave to explain the curcuits to other i want it as simple as possible
will goona try to make it on breadboard
thxx again

10. ### Gryd3

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Jun 25, 2014
Take a look at the schematic provided.
You can use a 9V battery with it just fine.
In a lit room, the photo-cell has a (usually very) low resistance, and with R2 will keep Q1 in saturation. While Q1 is 'on' it will be pulling the base of Q2 low which will keep it turned off. As soon as you move into the dark, the resistance of the photo-cell goes up, which will reduce the current flow into the base of Q1 hopefully enough to turn Q1 off. As soon as Q1 turns off, Q2 is free to saturate and turn on which will turn on any device you have connected to it

11. ### ayush soni

92
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Aug 24, 2014
hey i have a idea insted of using battery as power source c
can i use mobile charger
which i dont use
i mean i can cut its wire from where we plug in in to mobile
& i brought a LDR , some resistors & some led

12. ### ayush soni

92
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Aug 24, 2014
here is the circuit diagram

R4 is 10K resistor
R1 (LDR) 10K
R3(100 0hms)
T1 any low voltage NPN transistor
L1 is LED
2 1.5 volt battery

this should work
its simple & easy to make (i think )

but want u guyz suggestion

& also which transistor will be good ??? & any suggestion to make it better ?

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13. ### Gryd3

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Jun 25, 2014
Please note that the LDR will need to be less than 3kΩ to turn the LED off which may require quite a bright room. Increasing R4 will allow the circuit to switch on/off at a lower light level.

The + side should be at the top
The - side or ground at the bottom
The logical steps of the circuit should be from left to right. (Battery on the left, LDR on the middle/left, transistor middle/right, LED right. This is roughly the steps taken to get to the goal and it helps to draw it the same way)

14. ### ayush soni

92
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Aug 24, 2014
ok thxx
any suggestion ?

15. ### Gryd3

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Jun 25, 2014
Just to clarify, the LDR will act as a smaller value resistor in the light, and will be very close to its advertised resistance in the dark. R4 will be chosen based on the LDR you use AND how dark you want it to be. If you will be using a 9V source, you will need to make R4 and R3 a larger value.
I dont have a part number for you, but an NPN transistor will do for you.

16. ### ayush soni

92
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Aug 24, 2014
hm..
ok then i will use 2 1.5 volt battery

& i am little confused
will u plz post the proper value of R1,R3& R4 ?

sry for causing trouble
but m in soo much pressure here :'(

17. ### Gryd3

4,098
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Jun 25, 2014
The circuit you had made appears to me as though it will function for what you need.
The only change I would make if I had to build that circuit would be the addition of a potentiometer, or additional resistors for the experiment part of this.
I cannot give you a resistor value for R4 that will turn off when in a normal lit room because it is very difficult for me to determine where the LDR will be in a normal room without having one. It could be anywhere from 10K (perfect dark) to only 1-2K in bright direct light. With your current circuit, the LDR will need to be in bright enough light to make it drop below 3kΩ.
If you want to be able to set or adjust this. buy additional values for resistors to that you can swap out R4 for different numbers to find one that works well for you. (From 10k to 4.7k)
You can also put an extra resistor in-between the LDR and the negative battery terminal (something between 1-4kΩ) Both of these will adjust how dark/bright the room needs to be to turn on/off the LED. Using a potentiometer (variable resistor) will allow you to adjust the circuit while it is operating, so if you bring it to a different room, you can adjust it without swapping out parts.

18. ### ayush soni

92
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Aug 24, 2014
i have to show this in a room with normal lightning & i am thinking about using a flash light to increase intensity

& according to what u said R4 should be between 10k to 4.7k m i right & others value are
R1 (LDR) 10K
R3(100 0hms)
T1 any low voltage NPN transistor
L1 is LED
2 1.5 volt battery

& i think was thinking to use a 9v batter that what changes i have to do with resistors ???
and i think R4 is a variable resistor

i am not sure coz i download that image from a website

anyway for helping me
i love this forum

PS sorry english is not my first language ,so sorry for bad english

19. ### Gryd3

4,098
875
Jun 25, 2014
I thought you had drawn that image yourself. There is a circuit further up that would work with a 9V battery, you would only need to change the resistor on the LED to compensate.
If this project is for an electronics class, do you have any experience or knowledge working with transistors, or calculating voltage and current with resistors?

20. ### ayush soni

92
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Aug 24, 2014
lol i am a 17 year old kid & its not for electric class i want to make it for my exhibition i have to submit it after 24 hours
& dont have much time left