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Light Dependant Circuit

Discussion in 'LEDs and Optoelectronics' started by vick5821, Sep 20, 2012.

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  1. vick5821


    Jan 22, 2012
    Anything wrong with this circuit ?? I constructed it but seems not working. One more question, can I replace the 558 PNP transistor with 557 PNP ?? Thank you

  2. CocaCola


    Apr 7, 2012
    Yep, no power source...
  3. vick5821


    Jan 22, 2012
    got. The top right is + and tthe bottom right is negative :)
  4. MrEE


    Apr 13, 2012
    Based on this circuit, it looks to me that the intention is to turn the LED on when it gets dark. I recommend inserting a base resistor on Q1 (1k to 4.7k).
    A BC557 works as well. It just has a higher Vce rating (45 V, vs. 30V)
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The first thing I would do is insert a base resistor as suggested above (between the base of Q1 and the collector of T1).

    The next thing I would do is remove the LDR. The LED should turn on. If it doesn't then you may have wired something up incorrectly.

    Check very carefully that you have selected the correct transistors, they are in the right place, and that you have identified base, collector, and emitter correctly. These are probably the easiest things to muck up.

    What voltage is your power source?

    Incidentally, we would normally designate transistors as either T or Q, so you would have T1 and T2, or Q1 and Q2. This is a minor point -- since they do have different names and designations so it's not confusing (just odd).
  6. Terrance


    Feb 4, 2011
    my opinion is that you put R2 in between LED and LDR and then open the circuit and watch, is it work?
    i hope you like my opinion!
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