Light sensor help

Discussion in 'The Projects Forum' started by gt89, Feb 20, 2007.

  1. gt89

    Thread Starter New Member

    Feb 20, 2007
    Could anyone check this circuit and see if it would work, its for my project, and I need this light sensing circuit to have an output of 0v when its dark, and 12V when its exposed to light.

    Keep in mind, that the range of the LDR I will be using is 0.1Kohm(light) - 1Mohm(dark)


    If this doesn't achieve my specs, then can anyone direct me to a circuit that will?
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    o/p from the difference amplifier will be 11.2V with full light and 0.5V with no light. This assumes one has chosen an op-amp capable of single source operation.

    For this use, it would be more cost effective to set up the op amp as a comparator instead of a difference amplifier.
  3. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    No, this circuit will not work as desired.

    At full light and 100 ohms resistance, the voltage at the (-) input will be greater (more positive) than that of the (+) input, the output would be the difference inverted, or a negative output. Your negative supply rail is ground (not -12V), therefore the output will be above 0V because of the "headroom" required by the op amp.

    At full dark and 1Meg of resistance, the input difference is negligable and will not produce a significant output voltage (about the same as the "headroom").

    You may see a small amount of voltage change in the output as the light intensity varies.

    If you swap positions of the 100K resistor and the LDR you may have something that works, but the output will vary only from ~ +2V to ~ +10V if using a standard type op amp (not rail-to-rail).

    Also, look into using a comparator (LM311, LM2903, etc.) with a reference voltage on the (-) input.