So the problem is that I don't know how much resistors to add, or even if I should add some. I am using LED's and I always hear "Don't power LED's without resistance". I have been powering about 20 at a time in a simulation of arduino (supplies 4 to 12 volts) and it seems fine but a little dim. Do I need to add resistance? Allow more voltage? How do I add more volts?

My code:

void setup()
{
  pinMode(8, OUTPUT);
  pinMode(13, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(10, OUTPUT);
}

void loop()
{
  digitalWrite(8, LOW);
  digitalWrite(13, HIGH);
  delay(400); // Wait for 400 millisecond(s)
  digitalWrite(13, LOW);
  digitalWrite(12, HIGH);
  delay(400); // Wait for 400 millisecond(s)
  digitalWrite(12, LOW);
  digitalWrite(11, HIGH);
  delay(400); // Wait for 400 millisecond(s)
  digitalWrite(11, LOW);
  digitalWrite(10, HIGH);
  delay(400); // Wait for 400 millisecond(s)
  digitalWrite(10, LOW);
  digitalWrite(8, HIGH);
  delay(400); // Wait for 400 millisecond(s)
}

Cathodes are connected together and to ground
Anodes are connected to various pins
(Sorry for the minor incoherencies)

Moderators note: used code tags

 

If you want to run more than one LED at a time fron an Arduino pin, you really need a buffer of some sort. Having the Arduino switch a FET or transistor or driver IC is the way to go.
The Arduino pins can only supply a small current and if you push them to far you run the risk of popping the Arduino.

 

If you want to run more than one LED at a time fron an Arduino pin, you really need a buffer of some sort. Having the Arduino switch a FET or transistor or driver IC is the way to go.
The Arduino pins can only supply a small current and if you push them to far you run the risk of popping the Arduino.

Thanks soo much! I understand most of this and what a transistor is and how to operate one(for the most part) but can you show a simple schematic?

 

See if this helps..
https://electronics.stackexchange.c...using-a-minimal-number-of-pwm-pins-on-an-ardu