Help me please

Discussion in 'The Projects Forum' started by oolesh, Apr 21, 2012.

  1. oolesh

    Thread Starter New Member

    Apr 21, 2012
    24
    0
    hi everyone i got a premade clap on clap off curcuit so i want to MOD it by removing the led and then put a transistor 2N2222 to activate a 12 volt relay so i can put a bulb check it there http://www.mediafire.com/imageview.p...ajicuj37t7nbm7 btw do i need to put a resistor between A and B if yes what resistor do i need??sorry for my bad english PLEASE HELP
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,135
    3,054
    Yes, the resistor is needed to limit the base current to protect the transistor and your "signal". I think if you choose a resistor to drop 3-4v at the max rated base current for your transistor, you'll be fine. If the transistor fails to switch, you can back off on the resistance.
     
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  3. oolesh

    Thread Starter New Member

    Apr 21, 2012
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    thanks alot btw can i use a 1k ohm resistor on it??
     
  4. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    You can post electronic diagram assembly command that transistor 2N2222? After seeing the diagram, it may not be needed any resistance between points A and B, because between those points already has resistance in assembly joined.

    But if you do not post the electronic diagram, as a safety measure, you can mount a 1...4.7k resistor.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,135
    3,054
    It wouldn't hurt to start testing at 1k, which is probably too large, and then move down in value. As donpetru notes, you may not need it at all because there may be current limiting already on the circuit board, at the signal you are tapping. Do you know the voltage at "A" compared to your transistor's ground?

    BTW, you have shown any connection of grounds between the clapper and your battery. As drawn, your circuit really won't work unless there is a shared ground.
     
  6. oolesh

    Thread Starter New Member

    Apr 21, 2012
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    0
    i dont have any diagram sir so when i measure A and ground i got 3v what resistor value i need please ?? idk ohm's law :( my teacher is my internet :)! what is the min. volt to activate the 2n2222?? i think 3v is too high?? thanks a lot guys
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,135
    3,054
    For the transistor, the voltage (at this range) does not much matter, it's the current. Any voltage above about 0.6v on the base will begin to open the transistor. When it's fully open, the base current will be about 1/10th the current you are controlling. In your case, that's the current the relay is drawing. I don't believe you've told us that?

    The 3v you measured makes sense for the supply to an LED. And it means there is little danger in testing with resistor values at 1K and below. Do you have various values to test? It's very possible you won't need one at all but I'm sure you don't want to risk burning out your Clapper to find that out the hard way.
     
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