LDR characteristic

Discussion in 'General Electronics Chat' started by ecaits, Dec 16, 2014.

  1. ecaits

    Thread Starter Member

    Jan 6, 2014
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    Dear Friends,

    I am working on sun tracking system in which I am using two similar LDR to sense the intensity of sun by shadow method.
    But the characteristic of both similar LDR are different means at the same light intensity, both LDR shows different resistance.
    By using voltage divider circuit, I will detect the analog signal and provide to micro controller for comparison and accordingly I will operate the motor drive.

    To solve the issue I need to use two similar LDR which have same characteristic.

    Can anyone suggest me the LDR which have same characteristic or any other option?

    Plz find the attached images for reference.

    Thanks,
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    another way is to use led's, led's put out a voltage when exposed to light, if you use two led"s and put them in dark tubing, pointing a little awsy from cach other they will unballance their output depending which one gets most light.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Files that big are a problem for people with limited broadband connections. Please post smaller ones in future.
    If you have two LDRs of nominally the same resistance I would have thought that, with suitable geometry of the shading device and LDRs, any error due to differences between their characteristics would be fairly small and could be compensated (at least partially) by a fixed offset voltage adjustment in the circuit? What is your angular tracking tolerance?
     
    ecaits likes this.
  4. ScottWang

    Moderator

    Aug 23, 2012
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    Buy some others and pick two have the similar resistance would be better, otherwise you may trying to in parallel a higher resistor with one of them.
     
  5. ecaits

    Thread Starter Member

    Jan 6, 2014
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    Dear Alec t,
    The angular tracking tolerance should be less than 1 degree.
    Can you suggest me the standard LDRs which have almost same characteristic.?
     
  6. #12

    Expert

    Nov 30, 2010
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    CdS cells are antique! They don't work as matched pairs. I say you are using the wrong parts.
    See post #2
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    LDRs are far from precision - go on Ebay and look for some cheap enough to buy a good stock to select a similar enough pair from.

    The LDR contains cadmium sulphide - cadmium is extremely toxic, AFAIK; they're not RoHS compliant and most regular component suppliers have dropped them. There's still plenty coming out of China though.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    LDRs are very poorly matched. Use LEDs instead. Read this thread.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    LEDs can work as detectors as well as emitters - but I rather suspect that white ones are best avoided, as the phosphor might impede the light.

    The original white LEDs were blue with yellow phosphor, but I think they've switched to UV LEDs with white phosphor.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The LEDs I used in the referenced article were standard, ultra-bright Red LEDs that would normally be used as an indicator lamp (~20mA)
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    When used as light sensors are they likely to be better matched? Or would selection still be advisable?
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Can't ever recall having seen anything on an LED datasheet about its performance and specification as a light sensor, so I'd proceed under the assumption that these parameters are not guaranteed by the manufacturer.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    A quick search didn't find any direct comparison between red & green, but green seems also to be a popular choice.

    Also worth reminding the OP that water clear encapsulated types are better than coloured or diffused.
     
  14. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The little bit of testing I did suggests that the voltage produced by two randomly grabbed LEDs (out of a batch of 100) is within a couple of mV of each other (when measured with a high-Z dc voltmeter).
     
    Last edited: Dec 16, 2014
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    The sensor is going to be pretty high resistance, so an op-amp voltage follower isn't a bad idea - that would create various opportunities for trimming the transfer curve.
     
  16. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I do not understand why you would need closely matched LDRs in a sun-tracking circuit as you only need to know if one detects more light than the other. Just connect a pre-set pot between the LDRs and take a measurement from the wiper; adjust the pot until the output is 1/2 the supply voltage when the LDRs are equally illuminated. This method will only require one analogue input on the micro-controller.
     
    Last edited: Dec 16, 2014
  17. #12

    Expert

    Nov 30, 2010
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    Linearity. Cd cells can be adjusted to match at one brightness, but they will not track at a different brightness. Adjust in full sun and the first time a cloud is in the way, the tracker will run to one side or the other.
     
  18. ecaits

    Thread Starter Member

    Jan 6, 2014
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    Dear
    Dear Alfacliff,

    I am not getting exactly. Please elaborate in detail with diagram, if possible.
     
  19. ecaits

    Thread Starter Member

    Jan 6, 2014
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    Dear Sir,

    Have you any better option to solve the problem???
     
  20. #12

    Expert

    Nov 30, 2010
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    Yes. See post #2. It says to use LEDs as sensors.
    They should match and track much better than CdS light sensitive resistors.
    Set up a differential amplifier to find the difference in voltages created by the LEDs as they sense the light.
    Then design a dead band into the amplifier section so the motor doesn't "hunt" continuously.
    (As in the information to which post #8 refers.)

    Wait. You're using a microcontroller. Amplify the signals into a good working range of voltage. ADC the two signals, compare them, order the motor to move if one signal is "this amount" more than the other signal.
     
    Last edited: Dec 17, 2014
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