Help Needed With NPN Transistor

Discussion in 'The Projects Forum' started by MrScott, Aug 20, 2013.

  1. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    I am currently doing a project that requires me to be able to switch a 6v LED on when a 2v LED would normally be switched on. The 2v LEDs were bi-coloured and had a common anode (+).

    I have some 2n3904 standard NPN transistors and was hoping to use these but transistors are a shortfall in my electronics knowledge and I have tried several configurations but none have worked so far.

    I will be using a 12v power supply to power the new 6v LEDs in series with a 1K ohm resistor.

    Any help would be greatly appreciated.

    Thanks
     
  2. radiohead

    Active Member

    May 28, 2009
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    The 2N3904 transistor has three terminals: Base, emitter and collector. With the flat side (Assuming you have the TO92 case) is facing right, (Consult the data sheet for pin assignments). Hook it up so the +2V voltage on the 2V LED will be fed to the base (center pin) of the transistor, allowing it to conduct. Connect your 6V LED and resistor in series with the emitter (bottom lead) to battery negative. Connect the collector (top lead) to +12v.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Any sketch of what you have tried so far?
    Max.
     
  4. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    I have tried this already, the problem being that I need to have a different colour LED light up depending on which LED was originally lit up and because of the common anode with the original bi-colour LEDs, this does not work.

    Thanks for your reply.
     
  5. trader007

    Active Member

    Feb 27, 2010
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    if you have a common annode and 3 cathodes, don't you need a pnp? or likely, 3 pnp's?
     
  6. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    Would you be able to provide a detailed description of how to connect them up?

    Thanks for your reply.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What is the power source voltage? (what was powering the 2V leds)
    It could be as simple as just changing the resistor value.
    Or is this from a 5V microcontroller output pin?

    Also are these 6V led's bicolor too? or just 2 pins or what.
     
  8. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    The 2v supply is on a games console that I am making a new case for. I have removed the LEDs and measured the supply as 2v. I would not be able to change the resistor value as I have no precision soldering equipment.

    The new 6v LEDs are bi-colour with a common anode too.

    Thanks for your reply.
     
  9. ScottWang

    Moderator

    Aug 23, 2012
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    I'm not really understand what you are trying to do, but I still designed a circuit, I hope it may help you to talk what you want.

    [​IMG]
     
  10. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    I understand the circuit you have designed but unfortunately doesn't solve my problem (most likely due to me not explaining it properly.



    Currently 4, 2v, bi-colour LEDs with a common anode are switched on and off. I have removed these LEDs and in their place would like to put 4, 6v, bi-colour LEDs also with a common anode.

    I was hoping there would be a solution using 8 Standard NPN transistors as I have some lying around.

    Thanks for your reply.
     
  11. ScottWang

    Moderator

    Aug 23, 2012
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    Maybe you should attach your circuit and explan what you want to do, maybe we can modify the circuit to match for your need, otherwise, we only guessing, guessing can't solve your problem.
     
  12. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    This is the best attempt I can do at what I would like to happen. Please let me know if you would like something clarifying.

    [​IMG]

    Thanks
     
    Last edited: Aug 20, 2013
  13. ScottWang

    Moderator

    Aug 23, 2012
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    I still don't get it.

    How about the others as 4V,2V, 8 standard NPN BJT, Bi-colors LED?

    In the circuit only have 2V, do you want to input the voltage from B of bjt?

    If yes, then you just in series with a 1.5K on B, then when 2V input, the NPN will be on and turn on the LED.
     
  14. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    The circuit on the left is where the 2v LED used to be, one 2v bi-colour LED with a common anode.

    When the p.d. between 2v and GND 1 equals 2v then I want T1 to allow LED 1 to light up.

    When the p.d. between 2v and GND 2 equals 2v then I want T2 to allow LED 2 to light up.
     
  15. ScottWang

    Moderator

    Aug 23, 2012
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    I still don't understand what is the circuit used for?

    Why is the p.d. have to be 2V, and where does it came from?

    Have you consider the circuit that I posted at 9#, that is the same as you want two different LEDs light up in the same time, it just needs to change the two inputs, but I have to know what you want to do?
     
  16. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    In my earlier post (#8) I said that the 2v supply is on a games console that I am making a new case for. I have removed the LEDs and measured the supply as 2v. This is where the 2v is coming from and why it must be 2v.

    All I want to do is to replace the 2v LED with a 6v one.

    That's it, nothing fancy, I just cant do it directly so I was wondering if it could be done using a standard NPN transistor and if so, how.
     
  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Is your left drawing where the original LED's were ?

    What current do the new LED's need ?
     
  18. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    The circuit on the left is where one of the original LEDs were.
     
  19. tubeguy

    Well-Known Member

    Nov 3, 2012
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    What current do the new 6 volt LED's require ?
    Or, are these new LED's bright enough if you would hook them up using only the 12 volt supply and 1k current limiting resistor?
    To test the LED's you could use the circuit you posted above on the left provided you add a resistor connected to the base, other end to 12 volts. 10k would work.
     
    Last edited: Aug 20, 2013
  20. MrScott

    Thread Starter New Member

    Aug 20, 2013
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    20mA, The same as the old (2v) ones.
     
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