Ldr as light detector

Discussion in 'Homework Help' started by Hassaan Aslam, Nov 25, 2013.

  1. Hassaan Aslam

    Thread Starter New Member

    Apr 20, 2013
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    i want to use LDR as light detector with a transistor without using relay. my requirement is to get 5 v in day time and 0 volt in dark. that is i neet 1 and 0 state. what should i do????? :mad: :mad: :mad: :mad:
     
  2. MikeML

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    Oct 2, 2009
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    Post the specs for the LDR you plan to use, esp. Dark Resistance and Resistance Illuminated at a light level you want to call a ONE..
     
  3. Hassaan Aslam

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    Apr 20, 2013
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    I am a beginner i just want to make the sensor circuit, can use any standard value resistor....
     
  4. MikeML

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    Every LDR has a resistance vs light intensity curve. We need to have some idea of its resistance variation vs light intensity. Can you measure resistance with an Ohmmeter? If so, measure the resistance of your LDR in a dark room, and again with as much light as you want to detect. Post the two resistance values.
     
  5. takao21203

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    Apr 28, 2012
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    Bad circuit, transistor turns on slowly + heats up.
     
  6. MikeML

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    What circuit?
     
  7. takao21203

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    The one with a transistor. With the assitance of google it is not hard to find temp. controllers for fans or brighness evalution circuits based on OpAmp.

    It is not hard to make them working with given values.

    I don't discourage the discussion, I am just saying that one transistor approach will not give a good result.
     
  8. Hassaan Aslam

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    Apr 20, 2013
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    In room light resistance is 7.7kohm
    and in dark resistance is 164kohm
     
  9. MikeML

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    Ok, so you have a large resistance variation to work with. So you would like a Logic output, presumably with a bistable circuit which snaps from one state to the other.

    Have you studied a Schmitt Trigger circuit? That is a circuit that can be made with transistors and a few resistors that has a bistable output. What transistors do you have to work with?
     
  10. MikeML

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    Here is a link to suitable emitter-coupled Schmitt Trigger based on transistors.
     
  11. takao21203

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    Apr 28, 2012
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    yes the non-inverting comperator is shown.
    yes if OP wanted to, can be built discretely.

    But a chip is more flexible than a (calculated) transistor circuit.
    the transistor circuit will depend on the transistor parameters, maybe only a narrow range where it works as intended,...

    OP should feel free to breadboard, bag of transistors etc., but the knowledge it is not a good solution is not wrong.
     
  12. Hassaan Aslam

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    Apr 20, 2013
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    i have 2N2222 transistor
     
  13. MikeML

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    Do you have two of them?
     
  14. Hassaan Aslam

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    Apr 20, 2013
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    no only one
     
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