Dark Sensor LDR Circuit

Discussion in 'The Projects Forum' started by Rip, Apr 28, 2016.

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  1. Rip

    Thread Starter New Member

    Apr 28, 2016
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    How would I go about designing a dark sensor circuit using an LDR and transistors? The LDR value for light is around 7k ohms and when its dark it is around 1M ohm. The transistors are 2N3904. The circuit has a 5V buzzer and a 2V (10mA) LED. The circuit should work as follows: when there is sufficient light falling on the LDR, the transistor should be in saturation and the LED should be lit and when there is not sufficient light falling on the LDR then that transistor is in cutoff and the LED is not lit and the buzzer should sound. How would I go about designing this circuit? Any help is greatly appreciated
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
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    Typically it is a divider. Your transistor will have a threshold voltage to turn on / off. You just to pick a resistor so that

    LDRlight/(R+LDRlight) << Vthres, and LDRdark/(R+LDRdark) >> Vthres.

    In addition to that, you need to make sure that the base current isn't too light.

    A few trial and error will do.
     
  3. Rip

    Thread Starter New Member

    Apr 28, 2016
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    Where would I put the buzzer and the LED for this circuit? I'm just not sure how to lay it out to make it work

    Any help please?
     
    Last edited by a moderator: May 2, 2016
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    If this is a homework assignment, it should be in the homework section.
     
  5. bertus

    Administrator

    Apr 5, 2008
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