# Inverted LDR

Discussion in 'General Electronics Chat' started by adrian.dmc, Mar 4, 2007.

Feb 22, 2007
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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.

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2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Can you provide us with a part number so that we can look up the specific characteristics of the LDR?

hgmjr

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
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.

4. ### mrmeval Distinguished Member

Jun 30, 2006
833
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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.

Feb 22, 2007
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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.

6. ### hgmjr Moderator

Jan 28, 2005
9,030
214
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

7. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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What about using the LDR as part of a biasing divider?

Feb 22, 2007
52
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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).

9. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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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.