# 2n2222 LED base resistor

#### electronice123

Joined Oct 10, 2008
311
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?

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#### electronice123

Joined Oct 10, 2008
311
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?

#### c0de3

Joined May 1, 2009
50
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?
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.

#### Pencil

Joined Dec 8, 2009
271
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
resistor.

#### MrChips

Joined Oct 2, 2009
21,877
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.

#### electronice123

Joined Oct 10, 2008
311
Pencil,

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.

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