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Topic: How to decide the LDR 's (Photocell) external resistance ? (Read 6323 times) previous topic - next topic


Hi  everyone ,

I'm trying to design a sun tracking system with LDR (Photocell).Here's the circuit diagram which I refrenced : https://learn.adafruit.com/photocells/using-a-photocell

I want to figure out the external resistance's range, is there any function can help how to decide the resistance ?

Thank you,


You could measure it with a multimeter obviously, or take reading with analogRead and calculate it based on the value of the other resistor in your voltage divider.

Why do you care though? You're going  to get analogRead data from the divider somewhere in the 0 to 1023 range. All you need to do is find out by experiment what values correspond to the light levels you're interested in and then use them as thresholds to decide when to move your sun tracker. The actual value of the resistance needn't matter as long as you get a discernible difference in readings when sun conditions change.


For best accuracy the resistor should match the resistance of the sensor, near the middle of the sensor's "most useful" resistance range. However, the resistance of LDRs varies so much as a function of light levels that it can be almost impossible to choose a best value for the resistor. As wildbill suggests, pick a value that gives a good signal over the expected range of light levels, and you are finished.


The bias resistor should not be so low of a value to cause the LDR to self-heat!  Not only is it part of thee required voltage divider, but the resistor is a current limiter too.



    This sounds like an interesting project. I hope that you are still tracking this thread. I have an old style (huge) satellite dish that I want to turn into a parabolic reflector for a water heater. I need a solar tracker to run the motor on the dish. What was your plan? Please keep us posted as you make progress. Thanks, MT.


Short answer : read the datasheet of the LDR
That should have values for its resistance in dark conditions, but I'm not too sure how to convert datasheet info to its "full sunlight" resistance.  Best to take it outside on a sunny day with a multimeter.  uk full sunlight varies somewhat between 700 and 900 Watts per square meter.  Datasheet test conditions tend to prefer 1000 W/m^2 though they might show a chart or table.

I can't track the direction of variably bright sunlight with only one sensor but do record the signal indicative of the brightness of it landing on a scrap garden lamp 2V 3mA photovoltaic cell into a 270 Ohm resistor (smaller resistor to get <2V) and that provides a suitable analog volage for one of the "a0" analogInput pins.  I'll write that up if requested to., as the arduino a/d values might be closer to linearly dependent on sun power than what you are working on.


The resistor is require for matching the two LDR (East west or North South) so that when the sun is exactly at the top of both they both (LDR) gives same output.  (No two LDR's are identical they always give you slight difference in the Dark and Light conditions):

Following Formula was used:

LDR resistance(Ohms) = SQRT ( Rdark x Rnoon)

Where Rdark = Resistance of the LDR in night (No light condition) =247 Ohm (in my case)
          Rnoon= Resistance of LDR at high noon (direct under the sunlight) =39000Ohm (in my case)

LDR resistance = SQRT (247 x 39000)
                       = 3101 Ohm = 3.1 ohm
                       =3.2 is the nearest available resistance

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