Inverted LDR

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

  1. adrian.dmc

    Thread Starter Active Member

    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.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    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
    2
    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.
     
  5. adrian.dmc

    Thread Starter Active Member

    Feb 22, 2007
    52
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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
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    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?
    LDR.jpg
     
  8. adrian.dmc

    Thread Starter Active Member

    Feb 22, 2007
    52
    0
    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.
     
  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.
     
  10. adrian.dmc

    Thread Starter Active Member

    Feb 22, 2007
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    0
    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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