LDR circuit mod help needed

Discussion in 'The Projects Forum' started by holabr, Feb 3, 2011.

  1. holabr

    Thread Starter New Member

    Feb 8, 2010
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    I have an outdoor lighting transformer that is controlled by an LDR. Unfortunately there is no adjustment for sensitivity. I would like to modify the circuit (no schematic is available) to add a sensitivity pot. Based on some of the research I've done (I'm an electronics novice) it seems like I need a comparitor circuit of some type. What I'd like to do is have a circuit that would use the same LDR and provide 0 resistance when the desired light level is reached and very large resistance when it is dark. That way I can wire it in directly in place of the LDR without worrying about the rest of the original circuit (time functions etc.). Any ideas?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Have you considered "mechanical" means? By that I mean things like a light tube, partially shading the sensor, things like that. Could be much easier than modifying an existing circuit.

    If you must modify the circuit, try posting a schematic or photo of the circuit. Tough to speculate on solutions without that.
     
  3. holabr

    Thread Starter New Member

    Feb 8, 2010
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    The problem is it is too sensitive to "dark" right now. On a cloudy day my lights will come on. I really only want them on when it is dark outside.

    To get a picture, I have to take the whole unit apart. I did that once to see if I could find a solution. I took the LDR out of the circuit and played around a bit with it. That's how I discovered that if I shorted out the LDR or opened it I could mimic the "light" and "dark" conditions respectively. I was really hoping to have a possible solution before I take the whole thing apart again.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Well, the solution could be as simple as two more resistors, one in series with the cell and one in parallel. The resistor in parallel would reduce the total resistance and mimic light falling on the cell, causing it to see "dark" at a lower light level. The series resistor might be required to allow the total resistance to go high enough to trigger your circuit when the cell is truly in the dark.

    Getting the resistances right might take some experimentation. Using pots would simplify this, and maybe someone here could speculate on what range would be appropriate. One way to speed up testing would be to replace the cell with a pot, and determine what resistance is triggering the switch.
     
  5. holabr

    Thread Starter New Member

    Feb 8, 2010
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    Do you have any idea of what values I should start with? Does anyone else have a suggestion?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Is the sensor a CdS (cadmium sulfide) cell? Take a look at Google images if you don't know what one looks like.

    You could measure the response of this thing to light, and that would give you a good idea of what you need.
     
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