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Simple NPN transistor circuit help

Discussion in 'General Electronics Discussion' started by Mustwin351, Sep 29, 2014.

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  1. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Trying to use an NPN transistor as a switch but so far have had no luck.

    Are there are any problems in the layout of my circuit?
     

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  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes. Your battery is connected round the wrong way. Neither the transistor nor the LEDs will conduct with that polarity supply.

    Also your LED symbols are missing the arrows. That means they won't emit any light :)
     
  3. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Ok. I redid my circuit, tested and still nothing.

    I even added arrows to my leds and it still won't light!

    I thought on and NPN transistor you had the positive lead going to the base and the collector and the negative going to the emitter. Like in this image.
     

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  4. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Also this is my new schematic.
     

    Attached Files:

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    That circuit in post #4 will work. That is a PNP transistor in that diagram.
     
  6. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Thanks kris! Also found my other problem. Bunrt up resistor.

    My next goal is to do this little project.

    http://www.evilmadscientist.com/2011/does-this-led-sound-funny-to-you/

    ....but I'm thinking with my transistor being an NPN it will not work because of my "flicker" led having to connect to the Positive base of the transistor. Is my thinking correct?
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    what transistor do you really have ?
    you have shown 2 different circuits one using a NPN in post #3 and in post #4 a PNP

    you are confusing us and yourself ;)

    cheers
    Dave
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes, try to figure out exactly what you have.

    You can change a circuit from using a PNP to using an NPN, or vice versa, by reversing the direction of all polarised components (the power supply, all diodes, and any polarised capacitors).
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The circuit you linked is almost the same as in your post #4 and should work with the transistor you used there (PNP).
     
  10. Mustwin351

    Mustwin351

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    Apr 10, 2013
    The hand drawn sketches are my own and my transistor is specifically an NPN model # 2n2222a.

    I have my "flicker" led in the correct orientation as well and have confirmed this with the diode test on my multimeter.

    My voltage comes to 4.67 volts and I have a 100 ohm resistor to limit the current for the led.

    Why my flicker led won't light is baffling me.
     
    Last edited: Sep 30, 2014
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Make sure your circuit matches this one exactly (apart from the 2 ohm current limiting resistor).

    flickering led.png
     
  12. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Maybe they are white LED's you wouldn't see the light arrows from them on white paper :)
     
    Harald Kapp and KrisBlueNZ like this.
  13. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Ok I got it working....well partly...sorry it took me a while.

    The "flicker" led won't light (not that it's really a concern but it should be), and my other 2 leds are lighting but very dimly and are not flickering like they should.

    I've tried biasing the gate of the transistor without the flicker led and just used a ?????????

    ohm resistor but I get the same effect (other two leds dimly lit)

    My supply voltage is exactly 4.67.
    My resistor for the pair of leds is 25 ohms.

    Strangely though my multimeter says I am dropping 4v across the leds so I don't know why they would be dim. My transistor should be sourcing enough current and be fully saturated. Also I have the same 4v wether I'm reading across the two leds or the two leds and resistor.

    It also seems odd that across my flicker led and resistor used to bias the transistor I am only reading .5 volts. I would think I should get the full voltage drop of my source voltage. (4.67v)
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    LEDs require a certain amount of voltage across them before current starts to flow. Have a look at a typical VF vs. IF graph from a data sheet:
    LED IF vs. VF.png
    This graph shows how the voltage across the LED is related to the current that will flow through the LED. The LED brightness is roughly proportional to the current.

    If your circuit can't provide enough voltage to cause the current through the LED to be what you want, then the current will be lower than what you want, and the LED will be dimmer than you want.

    In your case, you have a 4.5V battery, and a transistor that will drop, say, 0.1V when conducting. That leaves about 4.4V across two LEDs in series. Voltages in series add together, so you will have about 2.2V across each LED. According to that graph, the LED current can't be more than about 2 mA.

    These numbers, and the graph, are not exactly the same as your circuit, but they illustrate the point. And the point is that you need a higher battery voltage.

    For more information, see Steve's tutorial at https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/
     
    Last edited: Oct 4, 2014
  15. Mustwin351

    Mustwin351

    49
    0
    Apr 10, 2013
    Thanks kris. According to the specs on my leds they each require only 2v a piece. I will try a higher voltage and see what happens.

    I confirmed I am using the yellow ones too
     

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