Simple Question On Wiring 8 5mm LEDs together

Using Arduino Project Guidance
#1

Hi Guys,

So I'm making a little LED light strip. According to the ebay auction I got the LEDs from, each 5mm LED is rated at 3v.

First question is, which is the better way to wire a light strip like this - in parallel or in a series?

Second question, is since this requires 24v (8 leds x 3v/ea = 24v), and the arduino can only supply 5v, I need an outside power source, correct?

To make sure I understand how this works, I have an external power supply that can supply 12v of power at 1.5amps. Each LED only requires 20mA. Since the LED's require 20mA and 3v of power, that means I need a current of 160mA. So I bring in a resistor to drop the current to manageable levels. 1500 mA - 160 mA means I should have at least a 1340 Omh resistor, correct?

So the resistor restricts the current from the dc power supply to only the 160mA all 8 LEDs require. But it would still blow the LEDs since the voltage level is still at 12v, right? So would I use a transistor here so the LEDs get only the required voltage, or a potentiometer to limit the voltage if I wired the LEDs in parallel?

Thanks for any tips.

#2

Well, resistors are pretty cheap. You can just put a resistor on each LED. But then the total current would be too much for a bare output pin. You would need a transistor, but you will need that in any case.

#3

Each LED requires a current limiting resistor. LEDs in series can share the same resistor. The supply voltage must be higher than the sum of the LEDs in series. Multiple circuits of that kind can be wired in parallel. The driver transistor must support both the supply voltage and the current sum of all parallel branches.

A PWM output can be used to further dim the LEDs, but it cannot replace the current limiting resistor(s) determining the maximal current/brightness.

#4