Light Dependent Resistors (LDRs) have resistance just like normal resistors, but the value of resistance they provide will vary by how much light is falling on them. I just tested a LDR and found that in extreme dark I could get the resistance into the millions of ohms, whereas in bright light (setting an headlamp LED on it) I was able to get the LDR down to about 30 ohms.
Make sure to limit the current through an LDR by having a resistor in series with it when it will be under bright light/at low resistance.
Light Dependent Resistor LDR component introduction for beginner learning electronics Click here to watch on YouTube!
Parallel resistor preventing light dependent resistor LDR from fully blocking current demonstration Click here to watch directly on YouTube.
Video below: Light Dependent Resistor LDR controlled LED intensity and 2N2222 NPN BJT on off switch circuits Click here to watch directly on YouTube!
A really common way to use the light dependent resistor LDR is to use it as one side of a voltage divider. The other side of the voltage divider typically consisting of a fixed resistor and/or an adjustable resistor in series with it to fine tune the voltage divider. The output voltage of this voltage divider can easily change to almost the full voltage difference of the power supply depending on how much light is falling on the LDR. The voltage divider output becomes the input of an amplifier that usually turns on and off at the required voltage.
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