# LDR detector

• posted

I want to have a LDR detector with variable range of light . let me explain more I connect a LDR to a LM2903 comparator and I want detect output in any condition ( in brightness and darkness ) but it not possible . I am amateur in electronic and I don't know what is wrong Please offer me a circuit for solve it . Thanks

• posted

You did not state the purpose of the circuit. Anyway you can try this:

Combine the LDR in series with a fixed-value resistor to make a simple voltage divider which can act like a voltage source that varies with light. This makes the first part of it. Let's call the voltage at the LDR V_Light. Now you have converted the resistance of the LDR into voltage which can be compared to a reference.

Now use a potentiometer to create a programmable voltage reference. You will use this to set the level that will trigger the comparator.Let's call this reference voltage V_Ref.

The point is to have the comparator compare two voltages. And now you have them, V_Light and V_Ref. All you have to do now is to connect V_Ref to the 'reference pin' on the comparator, you can connect it to the 'inverting input', next is to connect V_Light to the 'V_in', or just the 'non-inverting input'.

Going this way, the comparator will output a logic 1 if V_light is greater than V_ref, and a logic 0 if V_light < V_ref, and the other way around if you interchange V_Light and V_Ref (choose according to application). The behavior of the circuit depends on the trigger level you have set with the potentiometer, and the value of the resistor you have put in series with the LDR to form a voltage divider. You set the potentiometer according to the transition level between 'dark' and 'bright', this goes with trial-and-errors untill you get the satisfactory behavior.

The comparator needs a pull-up resistor. Connect the output of the comparator to the positive supply through a 3K resistor. Any close value should do.

Good Luck!

• posted

Dear friend ;

Thank you very much for your answer ; I used your idea and got good result Now I want advanced this circuit as I explain Imagine we want this circuit work in darkness and brightness automatically ! I mean when we change to sun light for example it can auto calibrate ref voltage . I explain more : In my project I use a laser beam that incidence to LDR and then I want detect it in op-amp output but it must work in room environment with normal light and sun light automatically without any adjust of potentiometer . in fact it must determine thresholds of voltage in both environment .is it possible or no ? I think I need a auto adjust voltage changer that can make threshold voltage May you help me ? Thank you and I'm sorry for my poor English=20 A=2Ejafry

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A. Jafry,

I think I understand what you are trying to do. You are trying to distinguish when a laser beam touches your LDR in either sun or low light conditions. You are looking for the "change" form a background light condition. Is this correct?

Ken

• posted

Modulate the laser, then AC-couple (and/or DC-restore) the receiver.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
• posted

Dear ken thank you for your attention . As you said this is right thing that I need . do you have any solution for it . Best regards A.Jafry

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Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

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Jim,

That's the way I was thinking, but without the modulation of the beam.

LDR and resistor as a voltage divider. Capacitively couple the juntion of the LDR and resistor to a high gain opamp. Only rapid voltage changes would be coupled to the amp. This would also assume a relatively stable or slowly changing light environment, with changes in the laser hitting the LDR as the major event. I did this once with an array of 8 IR phototransistors (and IR LRDs) all capacitor coupled to one opamp. I was trying to detect the daily activity of 3mm to 5mm blind cave a fish in a

1cm x 1cm x 8cm Plexiglas fish tank. Worked very well. The modulated beam, though a little more complex, would be better. I guess it would depend if A. Jafry is able to modulate his laser.

Ken

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ource that varies

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A. Jafry.

Since this is not a binary compatable newsgroup. I'll send you a sketch of the schematic to : snipped-for-privacy@gmail.com To maintain the detail in the sketch it is a large file (1Mbtye). Let me know if this a problem.

In the schematic your LDR would replace the Q8 phototransistor. You will have to experiment with values for C8, R8, and maybe the 100K and 10K resistors on pin 3 of the opamp.

If you have specific questions email me at

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a.Jafry,

The file is zipped down to 100K Bytes and sent.

Ken

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Dear Kenn Thanks a lot . I recived your file . I hope itcan solve my problem I send you resulat later Best Regards

Ken Moffett =D9=86=D9=88=D8=B4=D8=AA=D9=87 =D8=A7=D8=B3=D8=AA:

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