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I need to interface an LDR with a microcontroller.

While searching for some circuit ideas on the internet, I came across two different ideas:

  • One was using a voltage divider circuit with another resistor and input that into the ADC port of a microcontroller.

  • Another was to couple the LDR to ground via small capacitor and connect the other end directly to a microcontroller. (I am not sure how this works?)

    For example:

    schematic for measuring light using LDR without an ADC

    Image source: Measuring Light on a PIC 16F62x from Robot Builder

    The PIC 16F62X series is a pin-compatible upgrade to the old F84 offering lots of extra internal goodies such as timers, a USART and a couple of comparators. There are no analogue to digital converters (ADC) as such but it is a simple job to use a comparator, a timer and a few lines of software to make one.

Which one is more accurate?

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    \$\begingroup\$ Can you provide resource (scheme) for the second circuit. \$\endgroup\$
    – aiao
    Feb 13, 2013 at 12:46
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    \$\begingroup\$ accurate in what sense? what are you trying to accomplish? \$\endgroup\$
    – miceuz
    Feb 13, 2013 at 12:52
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    \$\begingroup\$ +1 on both above, but go option 1 if you have an ADC available. Very likely more accurate and much easier to understand / program. \$\endgroup\$
    – PeterJ
    Feb 13, 2013 at 12:56
  • \$\begingroup\$ @miceuz accurate for sun angle measurement. \$\endgroup\$
    – iamgopal
    Feb 14, 2013 at 2:41
  • \$\begingroup\$ robotbuilder.co.uk/Resources/Articles/149.aspx @aiao \$\endgroup\$
    – iamgopal
    Feb 14, 2013 at 2:44

1 Answer 1

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This is very simple:

Since both the voltage into the A/D input and the A/D's measuring range is relative to the PWR voltage, the PWR voltage cancels out. The A/D pin voltage will be non-linear with light intensity. Make R2 the value R1 has in the center of its range or where you want the most resolution.

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    \$\begingroup\$ @gopal - Just to add a little something: when I'm using a high-ish impedance sensor like this, I like to add a little capacitor (10nF - 100nF) between the A/D input and GND. This helps to charge the ADC's sample and hold capacitor more reliably, and also acts as a simple low pass filter. You'll get a better signal/noise ratio this way. \$\endgroup\$
    – Rocketmagnet
    Feb 13, 2013 at 17:15

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