Photoresistor limit setting – Light dependent resistor LDR

Light dependent resistors (LDRs), also called photoresistors, change there resistance based on the amount of light falling on them. When measured with my LCR meter, I found that my LDRs gets down to close to 0Ω of resistance under extremely bright light, and over 200 million ohms of resistance if it gets dark enough.



The ad above is where I got my LCR meter. It is not a replacement for a multimeter, but is a nice additional meter for getting more info about inductor, capacitor and resistor components than you can get from a standard multimeter.

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Min/max limit diagram

Light Dependent Resistor LDR schematic symbol with minimum and maximum limits diagram by Electronzap Electronapdotcom
Light Dependent Resistor LDR schematic symbol with minimum and maximum limits diagram by Electronzap Electronapdotcom
  • Series resistance will make sure that the total resistance will never fall below the extra resistance no matter how bright it is on the LDR. The added resistance will add up with the LDR resistance.
  • Parallel resistance will make sure that the total resistance never goes above the parallel resistance no matter how dark it is on the LDR.
  • Combined. You can put resistance in series with the LDR while also putting resistance in parallel with the LDR.

How to set LDR min/max limits video

Demonstrations of the circuit fragments shown above.

How to set a minimum and maximum possible resistance for light dependent resistor LDR

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