Just learning and confused what this resistor does

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

1. jasemilly

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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.

http://electricdiylab.com/make-ldr-darkness-sensor-circuit-simple-diy/

jasemilly, Nov 8, 2018
2. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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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.

(*steve*), Nov 8, 2018
3. Harald KappModeratorModerator

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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.

Harald Kapp, Nov 8, 2018
4. jasemilly

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Thanks I thought the LDR was able to do this on it's own.

jasemilly, Nov 8, 2018
5. BobK

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

Bob

BobK, Nov 8, 2018

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