Coursework Nightlight

Discussion in 'Homework Help' started by zain1612, Jan 21, 2010.

  1. zain1612

    Thread Starter New Member

    Jan 21, 2010
    2
    0
    Hi I am doing an electronics project and have decided to make automatic night light. To do I have used an L.D.R with a darlington pair so that Led turns off when light falls on it.

    [​IMG]

    Now for the development part of the project I have to explain why i used this circuit and thats where I need help as I used to crocodile technology to create I have no clue why i did what I did.

    Thanks in advance
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    There are two mistakes in the picture:

    [​IMG]

    Hope you understand the comments.

    Greetings,
    Bertus
     
  3. zain1612

    Thread Starter New Member

    Jan 21, 2010
    2
    0
    so i should have the leds on the same line and how should i arrange the darlington pair ?
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    The leds can be put in series with a corrected resistor OR parallel with seperate resistors.

    The darlington can be made by removing the indicated short.

    Greetings,
    Bertus
     
  5. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56


    Well shouldn't the LDR be connected to the positive side of the battery?

    Why would you need to have a transistor in the first place, why not just connect the LDR to the leds, or something like that, less components,

    What the heck are tore resistors on the base lead, looks kind of silly I mean what exactly are they suppose to do there, I don't know your whole circuit is so confusing, casn you explain to me why you have the emitter connected to the neg. term.
    Isn't it suppose to be connected to the positive side???

    Lioke I said the transistors in the circuit just don't make any sense, when you could just use the LDR and some leds...

    Sorry hope this helps.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    He wanted "dark" detector.
     
  7. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    Joni,
    Thanks,
    But I'm trying to make him think for himself...
     
  8. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    So as to not confuse anyone new to the forum, or who may come across this thread, the reply I gave above was not to be condenscending, or sarcastic, but was for this person to defend the purpose for making this circuit.

    Implied by this person, was they did not know how they came up with this particular circuit, how it works, but what I gathered was, this circuit, except for the darlington pair being a little off, minor mistake that was taken care of by the other replyer, this circuit configuration, was a good design, the thing that stood out the most was,

    THIS PERSON KNEW TO PUT THE LDR AT THE BASE OF THE TRANSISTOR, FROM GROUND and strategically between two resistors, to cause a significant voltage drop, that would drop the base voltage thereby shutting down the leds output.

    Very well thought out, for someone who did not know how the circuit workeed, and why they built it the way they did.

    My post was for this person to think and rebuttal on the stupid questions, I put forth, to be able to give a rebuttal at every question, I proposed, would have been all that was needed to understand the main concepts of the circuit.

    So to put any confusion to rest for anyone who comes upon this thread,

    Here is a rebuttal on the questions I proposed.
    --------------------------------------------------------
    "Well shouldn't the LDR be connected to the positive side of the battery?"

    No because then it would detect light and put a positive signal at the transistor base. A negative signal is what is needed, for a dark detector.

    "Why would you need to have a transistor in the first place, why not just connect the LDR to the leds, or something like that, less components"

    Again depending on the application, we want a dark detector, so we want a inversion of the input signal, input being light, we want output to go low.

    "What the heck are tore resistors on the base lead, looks kind of silly I mean what exactly are they suppose to do there, I don't know your whole circuit is so confusing, casn you explain to me why you have the emitter connected to the neg. term.
    Isn't it suppose to be connected to the positive side???"

    Those resistors at the base are for base bias, to put the transistor in conduction mode when NO light is pre4sent. But when a signal (light on LDR) comes in those resistors, produce a negative signal fed back to the base to cause the transistor to shut down, it's output...

    In using a NPN transistor the emitter is connected to the neg. term. to forward bias the base emitter junction. NOT the positive term.

    Hope this is enough to clarify any confusion that could have arose...

    Thankyou...
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Just a suggestion, but put a variable resistor between the LDR and the 5KΩ, say around 20KΩ. You can connect the transistor base to the wiper. This will allow the design to be adjustable for light intensity.
     
Loading...