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need help with LDR selection.

Discussion in 'General Electronics Discussion' started by markalishousfantastico, Nov 5, 2010.

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  1. markalishousfantastico

    markalishousfantastico

    10
    0
    Oct 5, 2010
    im making a light activated relay circuit that i want triggered by ambient light and the beam of a torch. it is needed to sense sunlight coming into a dark area when the door is opened or if the door is opened at night it also needs to detect flashlight beams. the current circuit is just a 2N2222 transistor with the collector attached to power and the collector of a phototransistor. the emitter of the phototransistor is connected to the base of the 2N2222 and the emitter of the 2N2222 is connected to one side of a 9V relay with the ground connected to the other side. the phototransistor however does not detect ambient light well so im looking to use an LDR. can the LDR be dirrectly substituted for the phototransistor and if so i looking at 3 different LDRs with parts numbers of VT93N1, NSL-5532 and NSL-4542. which one would be the best to use in my circuit any advice would be greatly appreciated.
    cheers
     
  2. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    I did something like this recently. If you want to specifically calibrate how much light needs to fall on the LDR before the relay is activated, then I suggest you use an Op Amp and a voltage divider.
    Based on how much the resistance drops(V+) with your ambient light intensity, you can adjust your voltage divider(V-) to be very close to it so even a few 100mV could trigger the relay.
     
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