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Just learning and confused what this resistor does

Discussion in 'General Electronics Discussion' started by jasemilly, Nov 8, 2018.

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  1. jasemilly


    Nov 8, 2018
    Hi I am trying to teach myself electronics etc have followed a few tutorials and made a few things now trying to learn a bit more on the theory side to understand better....

    I am looking at this webpage for turning on a light in the dark using a LDR, I believe I am ok with the majority of it but it shows a 50k ohm resistor before the LDR but no explanation or one that I can see that explains what it does. Why does the LDR need a resistor when it is one itself??

    thanks for all help and sorry if this is a silly question I have done some surfing around but didn't see anything that answered this.

    here is the link
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The 50k resistor and the LDR together form a voltage divider. The voltage at the junction of these two devices goes up in darkness, and down when illuminated.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The LDR and the resistor form a potential divider also called voltage divider. See here for a detailed explanation. Without that 50 kΩ resistor the base of the transistor would be at 9 V and the transistor would blow up as there is nothing to limit the currrent into the base. See also our resource.
  4. jasemilly


    Nov 8, 2018
    Thanks I thought the LDR was able to do this on it's own.
  5. BobK


    Jan 5, 2010
    Think of ot this way: the LDR is there to turn the light off, the resistpr is there to turn it on.

  6. Frankchie


    Nov 14, 2017
    If you put the LDR in place of the resistor then it could turn the transistor on and off "on it's own". The problem is that it would turn it on when the light is on and turn it off when there is no light.
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