Nightlite circuit

Discussion in 'General Electronics Chat' started by guruji, May 26, 2016.

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  1. guruji

    Thread Starter Member

    Nov 11, 2005
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    Hi guys would like to know if this circuit is good cause I am trying to make ldr more sensitive without success.
    Any help please?
    Thanks
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    @guruji , to what do you intend to connect the input of the bridge rectifier? Also, would you please post a more clear image of the schematic so we can read the values and possibly give you assistance?
     
  3. hp1729

    Well-Known Member

    Nov 23, 2015
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    Component values, please.
    What transistor? What photo sensor?
    What resistor values?
    What Zener diode?
    What LED?

    HP1729_nightlight.png

    You can probably pick one up at the local Dollar Store a lot cheaper and safer.
     
    Last edited by a moderator: May 27, 2016
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    LEDs to mains are a restricted topic, as per the Terms of Service.
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Design 762.PNG
    Quite correct, as it should be. So ...
    Will it work? Probably. Adjusting sensitivity? Not knowing anything about the light sensitive component all I can suggest is to adjust resistances until you get what you want.
    Now that we have the voltage down, the transistor can be just about any NPN you have on hand. 2N3904 is cheap.
    R1 - 150 or so
    R3 - 220 o 1K, adjusted for desired brightness
    R2 - depends or RT1
    RT1 - ???
    Zener - 1N751, or what ever you have on hand
    Transformer - At least 50 mA, or so, what ever you have on hand or can find. Or ...
    Base voltage will be V LED, + V R3 + about 600 mV = ? V
    As light decreases RT1 resistance increases until "? V" is reached and the LED turns on.
    What is the resistance of RT1 at that point? That gives you a value for current. Add 1 mA or so for base current. Then you know voltage and current and resistance for R2.
    adjust for sensitivity.
    I didn't bother to simulate or build it, but it should work.
     
    Last edited: May 27, 2016
  6. hp1729

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    Nov 23, 2015
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    Couldn't resist. I built it.
    Running from a 5 V supply ...R2 = 510, R3 = 220
    Works with red or white LED, so any LED will work.
    R1 depends on voltage source, figure 10 mA to 25 mA through it, or so.
    RT1 on hand has about 100 ohms in bright light and >1K in dim light. Typical values.
     
  7. boatsman

    Senior Member

    Jan 17, 2008
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    I found I can run a LED headlamp (12x 5mm white LEDs) off a Nokia telephone charger (5.3v 500 mA) without any modifications. All you need is a female connection to the original male connection.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Just a tad overkill for a night-light? ;).
     
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  9. DickCappels

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    Aug 21, 2008
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    After all this excellent designing, all we need to make it complete is for the TS to reappear...
     
  10. guruji

    Thread Starter Member

    Nov 11, 2005
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    Hi circuit is exact. That cds is an ldr I imagined yes?
    Is the zener important? what is it's function?
    I did a 1meg pot instead of the 150k but still ldr not sensitive enough.
    Any clue?
    Thanks
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    Try a smaller pot.1K or 5K.
    The working range of the LDR is down in the hundreds to thousands of ohms. No, the value of the Zener is not important. It just sets the operating voltage of the rest of the circuit. 5.1 V is just a suggestion. Actually when I built the circuit I just used a 5 V power supply. No Zener or R1.
     
  12. guruji

    Thread Starter Member

    Nov 11, 2005
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    Thanks for response. Yes but zener is controlling the voltage then of the circuit cause when I removed it leds stayed on.
     
  13. hp1729

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    Nov 23, 2015
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    Yes, removing the Zener throws operation off unpredictably. It just doesn't have to be 5.1 Volt. A little higher would still work. You just need to adjust calibration for the changes.
     
  14. guruji

    Thread Starter Member

    Nov 11, 2005
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    Today this circuit made an explosion when connected it to mains :0
    I've changred the transistor and made a 470ohm to a 4.7k pot. Don't know why maybe the transistor I chose was'nt the right one an r4001.
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you connected the post #1 circuit directly to the mains that's not surprising :eek:. Didn't you heed the post #3 warning and use a transformer as per post #5?
     
  16. guruji

    Thread Starter Member

    Nov 11, 2005
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    Hi Alec no I did not connected direct to BR. There is a 224j cap before br. I did something wrong for sure maybe switched transistor legs wrongly and blew piece of printed circuit like a fuse :(.
    Regarding transistor 100hfe better than 300hfe for this circuit?
    Thanks
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Do you mean you're using a cap as a voltage-dropper, instead of using a transformer? If so, that's a circuit contrary to the Terms of Service!
     
  18. guruji

    Thread Starter Member

    Nov 11, 2005
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    This was made like that; a chinese nightlite circuit with a capacitive transformer. I am trying to fix it.
     
    Last edited: May 28, 2016
  19. hp1729

    Well-Known Member

    Nov 23, 2015
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    Well, capacitor as an AC resistor. They use the capacitive reactance to drop the voltage.
     
  20. guruji

    Thread Starter Member

    Nov 11, 2005
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    Yes one of the problems is the 5k resistor ; I think it was too low so that was a sort of short.
     
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