Help with simple LED NPN transistor circuit

Discussion in 'The Projects Forum' started by Martialmedia, Jun 25, 2014.

  1. Martialmedia

    Thread Starter New Member

    Jun 24, 2014
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    [​IMG]
    I need help with the following circuit. It should light up when the two green wires touch, but it doesn't. It's actually going to be used to sense water. When the two wires are placed into water, the LED should light. So far it's a no go, and I can't seem to get it right. Any assistance would be appreciated.

    Thanks.
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
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    Each row on your breadboard is connected. You've shorted out the current limiting resistor for the LED, and it looks like your 4.7k resistor is going from base to emitter...

    Hook it up as shown here:
    [​IMG]

    Edit: see here
     
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  3. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Where does the far left green wire connect? One end of the 4.7k resistor appears to connect to the Base of the NPN (it's a little crowded on the picture in that area). Where does the other end of the resistor attach?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Looks to me as you have a few shorts, your breadboard has the rows of 5 that are all common to each other.
    I suspect you may have even blown the transistor.
    In a circuit such as that that requires a high impedance input, it may be better to use a Fet, 2n7000 etc?
    Max.
     
  5. Martialmedia

    Thread Starter New Member

    Jun 24, 2014
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    The other end goes to ground. In the image it is sharing the ground with the emitter. I have hooked it directly to ground as well, but it didn't work either.
     
  6. Martialmedia

    Thread Starter New Member

    Jun 24, 2014
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    Does that mean that the collector should just go to a power source and not also to ground?
     
  7. Martialmedia

    Thread Starter New Member

    Jun 24, 2014
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    Thanks for your replies. I did a bit of a redo, but still no luck. Here is the photo of the change.
    [​IMG]
     
  8. Richz28

    New Member

    Feb 8, 2011
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    On Row 14 the 100ohm resistor is shorted together.
     
  9. Alberto

    Active Member

    Nov 7, 2008
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    Move the 100 ohms resistor terminal along with the led terminal from row 14 to 15 ( as you connected the 100 ohms resistor is shorted and you have the led directly connected to the transistor collector and that is very bad!) Reduce the base resistor from 4.7 K to 1.5 K.

    Cheers

    Alberto
     
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  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As already mentioned early on, the columns of 5 are all one termination, (common).
    The collector, 1k res and LED are all shorted together!
    Max.
     
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  11. tshuck

    Well-Known Member

    Oct 18, 2012
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    Also, your base resistor limits Ib to ~0.5ma, meaning your transistor isn't getting to saturation (assume a gain of 10 to drive the transistor well into saturation...). Try a 1.5k base resistor...

    Note: fix the 100 ohm resistor first...
     
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  12. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Totally concur! If you make these modifications (including lowering to 1.5kΩ the 4.7kΩ resistor) and supply another picture, can you take one from both sides of the protoboard?

    (Amusing note: Autocorrect changed protoboard to pronto lard!)
     
    Last edited: Jun 25, 2014
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  13. Martialmedia

    Thread Starter New Member

    Jun 24, 2014
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    That worked perfectly! Thanks everyone for your help.
     
  14. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Congratulations!
     
  15. tshuck

    Well-Known Member

    Oct 18, 2012
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    Glad to hear you got it!

    Remember, protoboard for circuits, pronto lard for cooking! :)
     
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