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Need help !! LDR night light

Discussion in 'LEDs and Optoelectronics' started by Mr.K, Dec 15, 2016.

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  1. Mr.K


    Apr 28, 2016
    Hello there.

    I found a circuit guide to make a LDR night light. I have got a question :


    the 10K resistor reduces the brightness of LED quite remarkable. I tested with lower resistors , even less than 1K resistors. Brightness increases as resistor lowers , but the problem is that it also reduces sensitivity of LDR. I mean If I use a 1K resistor , for instance , then the LDR needs too much brightness of light to turn the LED off.
    What to do ?


    * I don't use a resistor between LED and power source (in the picture which is marked as "If needed")

    * I use 7-millimeter LDR . All other specifications are similar to the photo of circuit I attached

    Last edited: Dec 15, 2016
  2. Audioguru


    Sep 24, 2016
    Select a more sensitive LDR, they are all different. Mine have sensitivities of 10 to 1.
    Select a transistor with a higher hFE at high currents like a 2N4401 or a darlington transistor then the base resistor value can be higher.
    Use a resistor in series with the LEDs to prevent them from burning out.
  3. AnalogKid


    Jun 10, 2015
    You will get more even illumination and longer LED life if you have one current limiting resistor for *each* LED. Also, you are not going to get very "snappy" response from a single transistor circuit with no hysteresis. Consider two transistors, with at least one of them being a 2N7000 or 7002.

  4. KTW


    Feb 22, 2015
    I built a night light using an LDR enabled PIR sensor so it only operates when activated.
    It has to be quite dark to work.
  5. Mr.K


    Apr 28, 2016
    Thanks guys
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