Light monitor reading

I want to ask about values here. As i increases light readings that is our voltage increases or resistance i can say. But in theory of LDR i saw that as light increases resistance or voltage decreases.
So i am confused about its concept plz help me out!!


The basic theory of LDR is that when light falls on the LDR, the number of free electrons increases due to which resistance decreases.
For measuring the changes we are using a voltage divider circuit. You can refer the circuit diagram here for better understanding.
According to the voltage divider rule the higher resistance will have the higher share of voltage. Therefore when light falls on the LDR, its resistance decreases also decreasing the voltage across the LDR. Since, the voltage across the LDR decreases the voltage across the resistor increases which we are measuring using the analog pin.


Here we are measuring the output voltage across the 10k Ohm external resistor which is in series to the LDR sensor. The value will be between 0-1023 because of the analog pin.

The working principle of an LDR is photoconductivity, which is nothing but an optical phenomenon. When the light is absorbed by the material then the conductivity of the material enhances. When the light falls on the LDR, then the electrons in the valence band of the material jump to the conduction band. These pictures might help:

Variation-of-LDR-Resistance-with-Variation-in-Light-Intensity ------------ Light-Intensity-vs-LDR-Resistance

And if you aren’t familiar with Photoresistors:

A photoresistor is a type of resistor whose resistance decreases when the intensity of light increases. In other words, the flow of electric current through the photoresistor increases when the intensity of light increases. … Photoresistor changes its resistance only when it is exposed to light.

I hope it helps. Giving you a :heart: fella cause this is an interesting and crazy working principle of many similar sensors too.

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Okay you guys are correct but I think her question is more about the output we are getting as it differs from what we know about LDR.I have the same question as her because I am also confused when I keep the device in brightest zone it shows the highest reading that is 1.02 thousand and it confuses me. I would really appreciate if anyone could explain why is it acting like that.

The parameter that we have measured is the voltage. The analog pin reads the voltage and converts it into a value between 0 to 1023. If you see the circuit carefully you can see that we are taking the output voltage across the external resistor that we have used in the circuit. Since both the resistances are in series (LDR and the resistor) the voltage will be shared between both resistors depending on the resistance. You shine the light the light on LDR, resistance of LDR decreases along with voltage across it, decrease in voltage across LDR leads to increase in voltage across resistor which we are measuring. Therefore the more light you shine the higher reading you will get.
The value that you are getting when you shine the light is the voltage across the resistor. You are getting confused because you are trying to relate the measured voltage to the LDR.


Thanku so much I got it totally…

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Thank you ,this is the answer I was looking for

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thanks for the explanation in easy way

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I hope it helped to understand basic concept behind this