# How do you inverse the function of a LDR (Light Dependent Resistor) ....?

I know the schematic diagram ... But I don't know how it works ...
please can someone explain me the inverse function of a LDR
I've posted the schematic diagram also!

Thanks

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7 years ago
Swap the LDR and the resistor chain round, and the function will reverse.

This is very helpful! Thanks for the schematic now I understand!

KyleM32 years ago

I've also noticed that you can increase the sensitivity of the LDR. For me a 10k resistor was perfect. The led wasn't on unless it was truly dark. I even added a button to override-if the user wanted to press the button at any time with/without ambient light.

harshesh (author) 7 years ago
Hah .. right now ... with this diagram ... the LED lights when there is no light ... and is off when there is light ... and this is what i wanted .. I have tried building the circuit and it works completely fine!
this is the schematic diagram .. I got from one of the sites ...
And 'how the circuit works' is the thing I want to know ... whats the use of the transistor here (is it just to amplify the signal) ..the fixed resistor...  I have no idea .. and thats what i want to know ... I know the individual function of each of the components in this circuit ..
all i know in the circuit is that the variable resistor is too adjust the sensitivity ..
I want to know the working of this circuit ..

7 years ago
Think of the transistor as a switch. When the voltage across BE goes above a certain voltage it allows current to flow between C and E, powering the LED. The potentiometer allows you to vary the ratio of the potential divider and hence the level of light at which the LED turns on.

Read the articles at these links, they should explain how transistors work and how potential dividers work.

http://www.mayothi.com/transistors.html
harshesh (author)  Jayefuu7 years ago
Ohk .. I read .. So is it like this ?..

the transistor needs minimum 0.7volts to function ..
so we use it as a switch ...
The LDR on BRIGHTNESS.. LOWERS the resistance .. which in turn will LOWER the voltage drop across B and E of the transistor i.e. below 0.7V and the LED WON'T work ..
The LDR on DARKNESS ... INCREASES the resistance.. which in turn will INCREASE the voltage drop across the B and E of the transistor i.e. above 0.7V and the LED WILL emit light ...
I hope its correct ! :P
7 years ago
Correct!
harshesh (author)  Jayefuu7 years ago
Thanks a LOT mate! :)
Jayefuu7 years ago
So at the moment the LED turns on when it's dark. And you want it to turn on when it's light?

Explain to us your understanding of how it works at the moment. What's happening with the potential divider and how that affects the operation of the transistor. By doing that you'll understand it better than if we just told you what to do and if you can understand how it's working now you'll be able to work out how to do what you want. Yes?