2n2222 LED base resistor

Discussion in 'General Electronics Chat' started by electronice123, Feb 5, 2012.

  1. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    I have an LED pulsing circuit in which I am using a 4043 flip flop to turn on/off several 2n2222's which drive the LED's.
    How do I find out what resistor I need at the base of the 2n2222?
    Last edited: Feb 6, 2012
  2. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    Ok, I think I got it.

    Using multisim I used a single output from the 4043 to trigger 2 2n2222 transistors in order to switch some LED's. I put a 1K resistor on the base of each 2n2222 transistor and the simulation is working correctly...Using a 12V source.

    However, I still want to know, how do you determine the base resistor of a 2n2222 if the supply voltage is 12V and the transistor is switching a LED with a 470 ohm series resistor?
  3. c0de3

    Active Member

    May 1, 2009
    Im still learning myself but this is the best explanation ive found: http://www.electronicstheory.com/COURSES/ELECTRONICS/e101-47.htm

    The "more positive" discussion was helpful to me.
  4. Pencil

    Senior Member

    Dec 8, 2009
    As a general rule, for a transistor being used as a switch, the
    base current should be 1/10 of the collector current.

    If you post a simple schematic of what you are doing someone
    will be glad to explain in detail how to calculate the correct base
  5. MrChips


    Oct 2, 2009
    Let's say the LED drops 2V.
    With 12V - 2V in 470-ohm gives 20mA through the LED and transistor.
    If we assume a current gain of 10, the base current needs to be 2mA.
    Let's say 10V @ 2mA = 5K-ohm resistor.
    So 5K-ohm for the base resistor sounds reasonable.
  6. electronice123

    Thread Starter Senior Member

    Oct 10, 2008

    Using the 1/10 rule, if I have 25mA to my LED, then I want a base current of 2.5mA. Then the base resistor would be 12/.0025=4.8K.

    Sounds good, Thank you!

    BTW, the circuit is a simple 555, a counter and a 4043. The IC's are all powered by 12V. The 4043 outputs switch two 2n2222's which are connected to the 12V supply and an LED.
    Last edited: Feb 6, 2012