Inverted LDR

Thread Starter

adrian.dmc

Joined Feb 22, 2007
53
My question is if there is a way to make a circuit with a LDR that when there's no light it produces a lot of current, to light up LED, and when there is no light it produce very few current so that the LED doesn't light up.

I try to find LDR that worked backward that is:
  • when there is no light it has low resistance allowing big current
  • when there is light it has big resistance witch makes low current
But... I don't find it, so I try to make a circuit that inverted the effect of the LDR and after many tries the only thing that I could think of has the circuit of the attachment but I don't now if it is correct, and because I don't understand transistors I need some help.

Hope someone could help me.
 

Attachments

hgmjr

Joined Jan 28, 2005
9,029
Can you provide us with a part number so that we can look up the specific characteristics of the LDR?

hgmjr
 

beenthere

Joined Apr 20, 2004
15,819
There is not going to be such a device. LDR's work from the photoelectric effect - light on the CdS cell makes surface electrons more mobile, so an imposed voltage produces more current. There is no counterpart where the absence of light has a similar effect.

However, a circuit can be made where the CdS cell's increased resistance causes a transistor to become foreward biased, and so illuminate an LED. Note that the circuit will require a voltage source, unlike the one illustrated above, which has no hope of operating.
 

mrmeval

Joined Jun 30, 2006
833
You would need a circuit that inverts what the LDR is doing and controls the LED
You would use a comparator for that. It's possible to do it with discrete components or logic gates as well.
 

Thread Starter

adrian.dmc

Joined Feb 22, 2007
53
I was thinking in using a transistor, but I don't even know how to start so could you tell me how to do it, or could you give me a link to where I can learn how to do it, I have a very basic understanding of it but I don't know any formulas and
the things that it can make with it.

Thank you.
 

hgmjr

Joined Jan 28, 2005
9,029
We could still use the part number of the LDR you are using since they can have a wide variety of resistance characteristics and ranges.

hgmjr
 

Thread Starter

adrian.dmc

Joined Feb 22, 2007
53
I'm thinking in using that solution, but what values should the resistances have??

About the part number, I don't know it but let's pretend its the most used one (NORP12).

Thank you... in advance.
 

thingmaker3

Joined May 16, 2005
5,084
Resistor values will depend on how much light will be considered threshold, which transistor is chosen, the actual V+ available, and what LED is used.
 

Thread Starter

adrian.dmc

Joined Feb 22, 2007
53
About the considered light the operation of this system (if we can call it that) will be if we put the finger in top of the LDR the LED lights up if not it lights off.

About the V+ I think you mean the battery voltage if its that it will be 9V.

Finally about the LED it needs 3V/10mA (white or blue).
 
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