Wiring LEDs in series

Hi, I am very new to the Arduino software and to electronics in general. I was wondering how many LEDs I can wire in series to one pin on my Uno board. I am also wondering how to connect the lights and what sorts of resistors I would need.

I'm a first year college sculpture student, and this is for a project I'm working on.


I think the limitation will be voltage drop. Each LED (like any diode) has a voltage drop across it that depends on the semiconductor used (different for different colors). If the voltage drop is, for example, 1.5V then you could string three in series (4.5V) but not four (6V). The series resistor should ‘soak up’ the left-over voltage. In the case of three 1.5V-drop diodes that draw 20mA you’d need a resistor that will pass 20mA at 0.5V (5V - 4.5V). Ohm’s law gives the relation between Ohms, Volts and Amps so the calculation is easy.

If you want to get around the voltage drop limitation by putting strings of LED’s in parallel you will then have to worry about the current limit. Each pin of the ATMega processor has a maximum current it can source (when HIGH) or sink (when LOW). The ATmega168 is rated for an absolute maximum of 40.0 mA per pin. If your LED’s draw, say, 18mA you could put two strings in parallel (36 mA << 40 mA) but not three (54 mA >> 40 mA).

For completeness the typical forward voltages for LEDs are about 1.9V for red, 3.5 to 4.0 or so for green blue and white. Orange and yellow are intermediate. Thus you can drive 2 red LEDs but that's about it from 5V. It gets more interesting at 12V of course.

You can also wire 2 in parallel with current limit resistor(s) so the current in/out of the pin is not exceeded. I just completed a project where I had 2 RED LEDs in series on some pins and 2 BLUE LEDs in parallel on other pins. For REDs I used a 100 ohm resistor, and for BLUEs a 200 ohm resistor. These were the 5mm high brightness LEDs that dipmicro.com sells for 5-6 cents each.